In King County, Washington, penicillinase-producing Neisseria gonorrhoeae infections increased from 0.8% of reported cases of gonorrhea in 1986 to 6.8% of cases in the third quarter of 1987, then stabilized at 2.7% to 3.6% of cases. Of 268 penicillinase-producing N gonorrhoeae isolates tested, 159 (59%) belonged to a single clone, as evidenced by auxotyping, protein-I serotyping, plasmid analysis, and antimicrobial susceptibility testing. As this strain spread, the predominance of cases shifted from whites to blacks and from men to equal numbers of men and women. The proportion of cases associated with illicit drug use rose steadily from 19% in the first quarter of 1987 to 82% in the fourth quarter. Sixty percent of cases occurred in prostitutes or recent sexual contacts of prostitutes. These results suggest that core gonorrhea transmitters in King County are predominantly black illicit drug users, prostitutes, and their sexual partners. These are priority target populations for behavioral intervention and other measures to control the spread of all sexually transmitted diseases, including human immunodeficiency virus infection.