Four counties in Alaska and Florida have experienced the nation's most explosive growth since 1980, but the Southwest has taken in the most newcomers, the U.S. Census Bureau said yesterday.

The Matanuska-Susitna borough in Alaska was the only one of 3,138 U.S. counties to more than double its population, growing 119 percent between April 1, 1980, when the last major census was taken, and July 1, 1986, the bureau said.

The survey covers counties and what the Census Bureau calls county equivalents -- Louisiana's parishes, Alaska's census areas and cities in some states that are not part of any county.

Hernando County, Fla., on the state's booming Gulf Coast near St. Petersburg and Tampa, followed in percentage growth at 74.8 percent. Then came Kenai Peninsula borough in Alaska, up 70.7 percent; Flagler County, Fla., showing a 66.5 percent increase; and Gwinnett County, Ga., up 65.9 percent.

No county grew by more than 110,000 residents. For absolute population growth the winner was Los Angeles County, which grew 818,700 to reach 8.296 million. Both its numerical growth and the total lead the nation.

Maricopa County, Ariz., the home of Phoenix, came in second with 391,000 new residents, followed by Harris County, Tex., up 388,800; San Diego County, Calif., 339,500; and Dallas County, Tex., 276,700.

Loving County, Tex., was not only the biggest loser -- dropping 42.9 percent in population -- but also the least-populated county in the country, dropping to just 53 people as of July 1, 1986. Greenlee County, Ariz., had the next greatest loss, dropping 25 percent.

Then came Lake County, Colo., down 23.9 percent; Cibola County, N.M., down 18.3 percent; and Blaine County, Neb., down 17.6 percent.

In absolute population terms, the Rust Belt fared worst. Wayne County, Mich., where Detroit is located, led the nation with a decline of 173,500; followed by Allegheny County, Pa., the home of Pittsburgh, down 76,600; and Cuyahoga County, Ohio, where Cleveland is located, down 53,000. Seven states saw every county gain population: California, Connecticut, Delaware, Florida, Hawaii, Rhode Island and Vermont. In Iowa, 89 percent of the 99 counties lost people.