The three least populated U.S. states are Wyoming, Vermont and North Dakota, as of July 2009. These rankings are determined by the U.S. Census Bureau.
Wyoming is the least populated state with an estimated population of 544,270, followed by Vermont with a population of 621,760 and North Dakota with a population of 646,844. The District of Columbia has a population of 599,657, but it is not considered a state. Alaska, South Dakota and Delaware are ranked 47th, 46th and 45th, respectively.
The U.S. Census Bureau has population statistics on its website, Census.gov. Population statistics are found by clicking Topics on the home page and Population on the next page.