Experts say Sheriff Joe Arpaio's aggressive pursuit of illegal immigrants has contributed to the shift, The Arizona Republic reported Tuesday.
Critics of the controversial sheriff say he targets the poor and minorities. The Arizona American Civil Liberties Union has sued the sheriff's office for racial profiling.
"These numbers don't represent a pattern of crime, they reflect a pattern of enforcement," said Alessandra Soler Meetze, the group's executive director.
About 12 percent of the county inmates have immigration holds. Three years ago, voters in Maricopa County, where Phoenix is the county seat, approved holding suspected illegal immigrants without bail, while the county attorney ruled illegal immigrants could be charged as co-conspirators with those they paid to get them across the border.
Last year, the number of Hispanic inmates averaged 3,500, up from 2,751 in 2004. In 2004, there were 3,580 white inmates on average, which dropped to 2,938 last year.
Arpaio defends his policies and says the emphasis on holding illegal immigrants, almost all of them Hispanics, has put whites in the minority in the jails. The number of men held, 8,000 at any given time, has not changed for years.
"They're staying there. They're not getting the revolving door. When we arrest them, they can't get right out on bond," Arpaio said.