Telefonica of Spain moved to gain full control of Vivo, the largest wireless service provider in Brazil, with an offer of U.S. $4.1 billion to buy out its Vivo joint venture partner, Portugal Telecom (PT). Growth in wireless subscribership is strong in Brazil, which overtook Japan last year as the fifth-largest mobile services market in the world. Although Vivo leads Brazil’s mobile phone sector with a 28% share of that market, Telefonica Chairman Cesar Alierta contends that Vivo’s performance has suffered in recent months as a result of tensions between PT and Telefonica, which share control of Vivo with respective stakes of 50% each. In an interview, Alierta predicted that a purchase of PT’s stake would resolve that friction and boost Vivo’s operating efficiency. The buyout, if it succeeds, would also solidify Telefonica’s influence in Latin America, where it controls or holds stakes in carriers based in 13 countries. PT was given until August to respond to the proposal. Officials of PT offered no immediate comment.