He was the only candidate to the post. Fellow Liberals Algimantas Matulevicius and Romanas Sedlickas withdrew their candidacies at the Liberal Union executive sitting Nov. 9.
Steponavicius is also one of four deputies of Parliamentary Chairman Arturas Paulauskas, the only one that belongs to the opposition.
Steponavicius said he would only withdraw from this post if the parliamentary factions agreed that another Liberal MP take it on.
Steponavicius replaces former Prime Minister Rolandas Paksas as leader of the opposition. Paksas resigned on Oct. 27 after losing to Eugenijus Gentvilas in elections for the Liberal Union's chairmanship.
On Nov. 12, at the joint press conference in Parliament, Steponavicius and Gentvilas announced a new era in the Liberals' short history. "We will do more positive work, which was frozen because of the competition between Gentvilas and Paksas within the party," Steponavicius said.
Gentvilas announced this week that a full shadow government will be formed by the Liberal Union. Candidates to be shadow prime minister are Gentvilas, Steponavicius, Paksas and Vilnius Mayor Arturas Zuokas.