"The very first question she asked me was whether I was a legal resident here," said Mr. Carrillo, 20, now an electrical engineering student at Arizona State University in Tempe. "And I said, ‘Yeah, I am.’ And she said, ‘Oh good, that makes things easier."

Such questions have become commonplace in Arizona, where voters passed a 2006 referendum, Proposition 300, that forbids college students who cannot prove they are legal residents from receiving state financial assistance.

One of several recent immigration statutes passed by Arizona voters and legislators frustrated by federal inaction, the law also prohibits in-state tuition for illegal immigrants. Administrators at several campuses fear that the provision has priced some out of their classes, particularly at the state’s popular community colleges.

More than enrollment declines, however, what worries some educators here is that nonlegal residents — some of whom have lived in the United States since infancy and attended American high schools — will be afraid to pursue any form of higher education.

"The most frightening thing about the policy in place isn’t necessarily its measurable effect, it’s the immeasurable effect,” said Paul R. Kohn, the vice provost for enrollment management and dean of admission at the University of Arizona.

The law does not forbid nonlegal residents from attending college or require colleges to report them to the authorities, something the colleges have worked hard to convey. Still, supporters said the law would save the state millions of dollars and provide a powerful disincentive to prospective border-jumpers.