Spanish theologian and politician. He was born on 24th August, and belongs to a Carlist family. On September 1943, he joined the "San José de los Jesuitas" School of Durango. Two years later, Arzalluz moved to the apostolic School of "Javier" in Navarra, and did High School studies until 1949. The same year he joined the Oña seminary (Burgos), and in 1956, he started to teach Spanish Language and Literature at the "El Salvador" School of Zaragoza. Arzalluz began to study at the Faculty of Philosophy and Letters and of Law in 1957, in Zaragoza as well. When he finished Law, he was assigned to Frankfurt, at the Santk Georgen Jesuit seminary, to finish Theology studies. On 27th August 1963 Arzalluz was ordained priest. In 1965, after finishing the Theology degree, he was assigned to the "Santo Duque de Gandía" School in Valencia. During the school year 1966-1967, he worked as an assistant teacher of Professor Pablo Lucas Verdú at the University of Desuto. During the same year, he started a doctoral thesis named "The Democratic-Christian Union of Bonn" at the University Complutense of Madrid, without finishing it. On October 1969, he joined the Faculty of Political, Economic and Commercial Sciences of Madrid, in the Theory of the State department, headed by Professor Carlos Ollero Gómez. There, he published some articles in the Political Sciences Newsletter, edited by the Chair of Theory of the State: "The new German left-wing", "Extra-parliamentary opposition" and "Germany between two eras" between March and June 1970.

Arzalluz started to be interested in Politics, so he joined PNV (Basque Nationalist Party). On 7th December 1970, the Official Holy See Gazette announced his expulsion from the Society of Jesus. In 1971, Arzalluz got married with Begoña Loroño Bilbao, whom he met at the University of Desuto. The same year, he set up a law firm along with Santiago Eire Taboada and Javier Chalbaud. Arzalluz came back to the University, teaching Political Law at the Faculty of Economic Sciences of Sarriko, and then Law at the University of Deusto until 1977.

From the beginning of the 70's, Arzalluz sporadically kept contact with some members of the terrorist group ETA, such as José Luis Alvarez Emparantza or "Txillardegi". In 1971, Arzalluz was assigned member of the Regional Council of Bizkaia by Luis Mari Retolaza, and shortly after, he joined Euskadi Buru Batzar along with Retolaza, Sabin Zubiri, Luis Bergaretxe and Txomin Saratxaga. He was also the right-hand man of Juan Ajuriaguerra, the most important leader of PNV.

Arzalluz involved himself in PNV since that moment, gathering with some other politicians from the Spanish transition. During summer 1974, he participated along with Luis María Retolaza in a meeting with Rafael Calvo Serer, which was the representative of Democratic Board. After Franco's death, on November 1975, the political contacts intensified. On October 1976, Arzalluz took part in the meeting in which Luis María Retolaza and Mikel Isasi gathered with Rodolfo Martín Villa, Minister of Government, and José Ramón Onega, Director-General of International Politics. There, they established the plans to carry out the political transition in Spain.

In 1977, he was elected deputy of PNV for Guipuzkoa, and during the following years, he fully involved in the Spanish Parliament, where he represented the Basque Minority at the Congress. On January 1978, he formed a team along with Juan Ajuriaguerra, Carlos Garaicoetxea, Marcos Vizcaya, Juan José Pujana and Pedro Sodupe , who handled some amendments to the draft bill of the Spanish Constitution, elaborated by the Congress. On 20th June 1978, the seven parliamentary groups agreed an amendment for which the historical rights of regional territories were protected, and they proposed a general updating of that system. The PNV abstention to the Constitution would be highly relevant in the future. Juan Ajuriaguerra's death and his following replacement would decisively affect to this process. At the beginning of July 1979, Arzalluz leaded the nationalist group in charge of the talks about the creation of the Autonomous Statute, accompanied by Carlos Garaicoetxea, Emilio Guevara, Jose Luis Cuerda and Juan Mari Ollora. Arzalluz was re-elected in the 1979 elections, but at the end of this year, he left to join the Executive Board of PNV in Bizkaia. On June 1980, with Carlos Garaicoetxea's appointment as Lehendakari or the Basque regional president, Arzalluz moved up and strengthen within the party. From the end of 1978, PNV underwent an important internal crisis, facing the conservative, independent and abstaining line, leaded by Antón Ormaza, and the line influenced by European Christian democracies and Social democracies, defended by Xabier Arzalluz. The expulsion of the local leaders from Bermeo constituted one of the most relevant stages within the PNV. Between 1980 and 1982, Arzalluz took part in the commissions of PNV which made contact with ETA. On 18th January 1980, after passing the Guernica Statute and before the first elections of the Basque Chamber, Arzalluz and the rest of nationalist deputies left the Congress of Deputies to protest against the Government autonomous politics. The internal crisis in PNV was triggered with the development of the Law on Historical Territories, which had a strong opposition within the party. Arzalluz's position, which was favourable about keeping the personality of the three territories that make up the Basque Autonomous Community, faced the Garaicoetxea's proposal about reinforced the importance of the Basque Government. On 16th May 1984, Arzalluz resigned from Euskadi Buru Batzar and moved to England in order to study English at the University of Cambridge. The PNV crisis was finally triggered by the entry of Spain into the Common Market, the favourable position about NATO and the future pact with the socialists. Garaikoetxea tendered his resignation as the president of PNV, and José Antonio Ardanza was presented as the new leader by the party. Arzalluz came back to Euskadi on 15th October, and became professor in Political Law at the University of Deusto. On 25th February 1986, he took over again the presidency of PNV.

On January 1988, Arzalluz closed the PNV conference at the Arriaga Theatre in Bilbao, which had great media transcendence because of its self-critical nature, so it will be known as "Arriaga's Spirit". On 12th January 1988, Arzlluz signed the "Ajuria Enea Pact" on behalf of PNV, which marked a relevant period of moderation.

On November 1996, PNV and Partido Popular, headed by José María Aznar, reached a new agreement. Herri Batasuna's (Basque left-wing nationalist party) political isolation, proclaimed by the President Ardanza, triggered a new period which ended with the "Lizarra Pact", on September 1998, and the creation of Udalbiltza, which was a Basque Municipalities Assembly. The controversial moderate nationalists' role about the pacts established with ETA before 16th September 1998, when the terrorist group announced the declaration of truce, increased the tension between nationalist and constitutionalists groups. The pact between PNV-EA and Euskal Herritarrok, a new political group created because of the joining of some other groups into HB's proposal, was highly decisive within this process. During the electoral campaign before the May 2001 autonomous elections and the breakdown of ETA's ceasefire, Arzalluz signed along with Bloque Nacionalista Galego (Galician nationalist party) and Convergencia y Unió de Catalunya (Catalonian nationalist party) the "Pact of Barcelona" on 18th September 1998. It was a document in favour of the establishment of a state consisting of several nationalities. Arzalluz was re-elected president of PNV on January 2000, resigned from it in 2004.