Florida, with a 23 percent population increase so far this decade, has surpassed Pennsylvania to become the country's fourth most populous state, the Census Bureau said yesterday.

According to 1987 bureau estimates as of July 1, Florida's population increase of 2.3 million trailed the gains made in only two other states: California, with an increase of 4 million, and Texas, which gained 2.6 million.

In the state rankings, California, with 27.6 million people, is first, followed by New York, with 17.8 million, and Texas, with 16.7 million. Florida takes fourth place with 12 million. And Pennsylvania, whose population is estimated at 11.9 million, slips to fifth place.

The bureau also reported that Alaska had the largest percentage population growth from 1980 to 1987 -- nearly 31 percent, from 402,000 to 525,000.

Other states posting fast growth rates in the 1980-87 period include Nevada, which experienced 26 percent growth, and Arizona, which had a 25 percent growth rate. Nevada's population now totals 1 million while Arizona has an estimated population of 3.3 million.

Overall, the bureau said, the South and West experienced most of the country's growth in the 1980s, accounting for 15 million of the 17 million population increase.

The South has grown 11.3 percent this decade and now has 83.8 million people, and the West grew 15.1 percent to 49.7 million, according to bureau statistics.

The bureau said all of the industrial midwestern states have experienced population gains from 1986 as the regional economy has continued to recover from the recession in the early 1980s.

But some farm states -- Iowa, Nebraska, North Dakota, Montana and Idaho -- had another year of losses, as did the energy-depressed states of West Virginia, Oklahoma and Wyoming, the bureau said.

Louisiana, an oil state suffering economic ills, for the first time experienced a population loss from the previous year's estimate, dropping 38,000 to 4.46 million.