BERLIN  The European Commission opened investigations Monday into whether I.B.M. had abused its dominant position in mainframe computers, signaling that the era of aggressive prosecution of American technology leaders in Europe did not end with the Microsoft antitrust case.

The commission said that it would examine whether I.B.M. had shut out rival mainframe software vendors and service providers. The investigation could lead to charges and potential fines against the company.

Joaquín Almunia, who took over in February as the European competition commissioner, “is making it clear that the Internet and the information technology sector will be an enforcement priority for him and for the commission,” said Alec Burnside, a competition lawyer in Brussels at the law firm, Linklaters. “It is clear that he is building on the precedent left him by his predecessor.”

Mr. Almunia’s predecessor, Neelie Kroes, oversaw the conclusion of a 10-year investigation against Microsoft, which eventually required the world’s largest software maker to pay 1.2 billion euros, or nearly $1.8 billion, in fines and to change its business practices in Europe.