Though community residents often refuse to face up to it, rowdy bands of youths have staked out new turf - suburbia.

The rival gangs piled into one another with a vengeance - fists flew, knives flashed, clubs struck muscle and bone with sickening smacks. When it was over, a 19-year-old youth lay dead.

No, the scene was not a dismal dead end street in New York, Philadelphia or Los Angeles. The gang fight occurred in Evanston, 111., a mostly well-to-do suburb north of Chicago.

Evanston's problem is far from a unique one. Street gangs - once confined to the slums of the country's biggest cities - are found increasingly today in smaller cities and suburbs as well.

Federal researchers discovered that two-thirds of the cities reporting street-gang problems had populations below 500,000.

Out of the core.The spread of gang activity from the inner city began more than two decades ago in California. Today, of an estimated 28,300 gang members in Los Angeles County, 20,000 live outside the city of Los Angeles.

Although the pattern established in California is not as pronounced elsewhere, it is growing fast in some areas. Law officers report that at least 20 Chicago suburbs have youth-gang problems nowadays. Five gangs, with 400 members all told, compete for turf in Evanston, population 73,000. The once peaceful suburb was shocked by two gang-related murders.

Cicero, a blue-collar Chicago suburb of 60,000, also numbers street-gang members in the hundreds and was the scene of two gang killings last year. In East St. Louis, 111., an impoverished city of 55,000, investigators blame gangs for three recent murders and the firebombing of a police officer's home.

Youth gangs are spreading in part because the conditions that spawn them in the old urban cores are becoming more prevalent in suburbs and small cities, experts say. They point to racial and ethnic separation, poverty, family breakups, high youth unemployment and lack of recreational activities.

"Psychological denial." Gang activity is often transplanted to new areas by juveniles whose families fled the inner city. Some of the gangs of Phoenix, for example, were started by youths who moved there from Los Angeles, police report. In the same way, offshoots of Chicago gangs have appeared in Indiana, Iowa, Wisconsin, Minnesota and Michigan.

Suburban gangs often thrive because the community refuses to admit it has a problem. "There's psychological denial," says Dennis Rosenbaum, a criminologist at Northwestern University. "People move to the suburbs to get away from those kinds of problems. The business community, the political community and police all have vested interests in denying gangs exist. Real-estate values are at stake."

Whether in big cities or small, many gangs are turning more violent.

"Instead of supervising hubcap stealers, the probation officer is supervising children who committed burglary, armed robbery and sexual assault."