Union membership showed its largest annual increase in more than two decades, but the number of American workers who belong to a union remained static as a percentage of the nation's growing work force, the Labor Department reported yesterday.

The department's Bureau of Labor Statistics reported that union membership registered a net increase of 265,000 in 1999. Union workers now total 16.5 million, representing 13.9 percent of the work force.

It was also the first time since 1978 that union strength in the private sector did not decline, holding statistically steady at 9.4 percent of the work force. Unions represent 37.3 percent of government workers.

The private sector gains came despite the loss of 100,000 union jobs in manufacturing, with much of the new growth in construction and service-sector jobs.

The report also showed that last year union members had median weekly earnings of $672, compared with a median of $516 for workers who did not belong to a union.

"This is all very good news," AFL-CIO President John Sweeney told a news conference. He said the numbers showed the federation's organizing efforts were beginning to have an impact. "These numbers show that we're turning the corner, but we're not at our destination yet."

Sweeney acknowledged that "some of the growth is due to overall job growth," but he said more of the union growth last year was from organizing efforts by individual unions.

The AFL-CIO, which has 13 million members, said its own internal data shows that at least 600,000 workers organized unions in 1999, a 25 percent increase over last year. Just how the 600,000 fits into the government figures is unclear, however. The AFL-CIO does not count people who join unions as members until they are covered by a collective bargaining agreement, a process that often takes more than a year after a workplace is organized.

As a result, the majority of the 600,000 are not counted in the 1999 membership statistics, but the AFL-CIO is not sure precisely how many have been counted.

Kirk Adams, organizing director at the AFL-CIO, estimated that because of the lag between organizing and gaining a first contract, the labor movement would need to sign up 500,000 to 1 million new members a year just to keep the pipeline full so that annual membership would continue to show a growth rate.

In recent years attrition has forced labor to sign up a minimum of 400,000 new members a year just to stay even with the previous year's membership level.

African American men are more likely to belong to unions, with a membership rate of 20.5 percent. White and Hispanic women have the lowest membership rates at 10.9 percent and 10.4 percent respectively. As a percentage of overall union members, African Americans account for 17.2 percent, whites 13.5 percent and Hispanics 11.9 percent.

In what could prove to be bad news for labor unions in the future, the Bureau of Labor Statistics said that "workers ages 35 to 64 were more likely to be union members than their younger counterparts."