In an attempt to fight identity theft, Penn State University is ending a long-standing practice of using Social Security numbers to identify students.

A new nine-digit number went into effect Monday. Penn State was advising students and employees to keep their old ID cards with Social Security numbers until they were sure the new cards would work for all purposes at the highly automated university.

Penn State officials would not estimate the cost of the conversion for its more than 80,000 students, thousands of employees and records for its half-million former students.

"Colleges and universities should have taken this step several years ago," said Linda Foley, co-executive director of the Identity Theft Resource Center in San Diego.

"Because they didn't, college students are one of the three segments of the population most at risk of becoming victims of identity theft, along with senior citizens and military personnel," she said.