Gonococcal infection in Colorado Springs, Colorado, is concentrated in about 1% of the population. The social groups at risk are characterized as young, nonwhite, heterosexual, and connected to the military. They exhibit residential proximity by clustering in "core" census tracts; 51% of cases were in four tracts. They demonstrate residential stability and close social association at preferred sites for nighttime leisure activity (six major sites out of 300 available). Social aggregation is further demonstrated by the length of social contact prior to sexual contact (45% had known each other for over two months), the neighborhood nature of sexual choices, and the grouping of sexually connected individuals in lots (six lots contained 20% of cases). The force of infectivity, measured in person-days of potential spread of gonorrhea by infected contacts, provides a quantitative assessment of the important of identifiable social groups in the transmission of gonorrhea.