The Census Bureau said Maricopa County gained 696,000 residents from 2000-2006. The county has 3.8 million residents, making it the fourth largest in the United States, with some 3 million of those added since the 1970 Census.
Harris County, Texas, which is home to Houston, posted the second largest growth, adding 486,000 residents for a total population of 3.9 million.
The Census Bureau said three of the 10 fastest growing counties were in Texas and three were in California while the others were in Arizona, Georgia, Illinois and Nevada.
Among areas that saw the number of residents drop, Orleans Parish, La., hit hard by Hurricane Katrina, lost 261,000 residents from 2000-2006 and Wayne County, Mich., saw a drop of 89,000. Cook County, Ill., lost 88,000 residents but is still the second largest county in the country.