Nearly two-thirds of the 5.4 million legal immigrants from Mexico are not citizens of the United States, even though they are eligible to naturalize. Their rate of naturalization——36%——is only half that of legal immigrants from all other countries combined, according to an analysis of Census Bureau data by the Pew Hispanic Center, a project of the Pew Research Center.

This naturalization rate of Mexican immigrants in 2011 compared with 61% for all immigrants and 68% for all non-Mexican immigrants. The naturalization rate of Mexican legal immigrants is also lower than that of immigrants from Latin America and the Caribbean——36% versus 61% in 2011.
Read more information here