Legal immigrants in the United States have opted to become American citizens in historically high numbers in the last decade, according to a study published yesterday by the Pew Hispanic Center.

The number of naturalized citizens in the United States population in 2005 was 12.8 million, a record high, the study found. The number reflected not only a growing number of legal immigrants coming to the United States, but also a growing eagerness among them to adopt American citizenship, the study reported.

Of all the immigrants now living legally in the United States, 52 percent are naturalized citizens, while others are permanent residents or have some other immigration status. The last time that citizens made up more than half of all legal immigrants was in 1980, the study found.

The research, which is based primarily on census data, was done by Jeffrey S. Passel, a demographer at the Pew center, a nonpartisan organization based in Washington.