In the past, shigellosis in Seattle-King County has been primarily a disease of children, their parients, and foreign travelers. During the 18 months beginning in July 1975, an outbreak of shigellosis in Seattle's community of gay men involved both Shigella flexneri and Shigella sonnei. They accounted for nearly 30% of all cases of shigellosis reported to the health department. Fellatio and/or oral-anal contact was reported by 90% of the infected homosexual men; this was probably the mechanism of transmission of most infections. Intercity spread was determined by case histories and by the finding of S. flexneri 3, a previously unusual organism in Seattle.