Data were obtained from the Compressed Mortality File, a county-level mortality and population database maintained by the National Center for Health Statistics, Centers for Disease Control, Hyattsville, Md. The setting involved 80 counties with a population of at least 10,000 black males 15 through 19 years old in 1987 through 1989. From 1983 to 1985 and from 1987 to 1989, seven and 13 counties, respectively, were identified with significantly high firearm homicide rates. Firearm homicide rates were significantly high in both time periods in the following counties: Los Angeles, Calif.; Wayne, Mich.; Kings, N.Y.; St. Louis City, Mo.; and Baltimore City, Md. Firearm homicide rates increased significantly between 1983 through 1985 and 1987 through 1989 in 34 of the 80 counties. Twenty counties had significantly low rates in both time periods. Several counties with low rates from 1983 to 1985 experienced significant increases and by 1987 to 1989 were among those with high rates. The researchers advise that surveillance of firearm homicide rates at the county level in counties with high and low rates is a necessary first step in the development of effective violence prevention programs. Those counties where rates are high and increasing are the counties that should be targeted for intervention strategies. Knowledge of the incidence of nonfatal firearm injuries is also needed. 5 figures, 1 table, and 3 references