Only about a third of the 5.4 million Mexican immigrants who are eligible to become U.S. citizens have done so, a rate much lower than that of other legal immigrants, according to a report released Monday by the Pew Hispanic Center.

Mexicans are the largest pool of eligible immigrants, accounting for almost a third of the 12 million legal permanent residents in the United States. But while 68 percent of eligible non-Mexican immigrants have become citizens, only 36 percent of eligible Mexicans have, said the report, titled "The Path Not Taken."

The study was based on data from the U.S. Census and the U.S. Bureau of Labor Statistics and on a nationwide bilingual telephone survey of 1,765 Latinos, more than half of whom were immigrants.

The low rate of Mexican naturalization is not from lack of interest: 94 percent of Mexican legal permanent residents said they would become citizens if they could. They cited barriers such as a lack of proficiency in English or fears of not passing the citizenship test, and the $680 application fee, which is nearly double what it was until 2007.

Around 12 percent of Mexican respondents said they simply had not gotten around to applying.

The report comes as the debate over immigration has heated up. Last week, a bipartisan Senate group proposed to create a path to citizenship for most of the 11 million unauthorized immigrants living in the United States, and President Obama said he would put forth a similar bill if necessary.

"The question is, how many of them may take advantage of that?" said Mark Hugo Lopez, one of the report's authors. "This research shows that among Mexican immigrants who are eligible to become U.S. citizens, relatively few do when compared to other immigrant groups."

About 61 percent of legal immigrants from other parts of Latin America and the Caribbean are naturalized, the study found.

The nationalities with the highest rates of naturalization - about 75 percent - are Vietnamese, Russian, Filipino, Korean, Laotian and Cuban, Lopez said. Nationalities with the lowest rates after Mexican - about 40 percent - are Guatemalan, Honduran and Nicaraguan.

One possible reason for the lower rate of naturalization among Mexicans is that they are more likely to maintain close ties with their homeland because of its proximity, the report said.

The last time that the U.S. government created a path to citizenship was in 1986.