The seroprevalence of antibodies to simian immunodeficiency virus (SIV smm) and simian T-cell leukemia virus type I (STLV-I) in a captive breeding colony of sooty mangabey monkeys was determined, and infection by SIV smm was confirmed in all cases by virus isolation. Among 138 animals tested, 57 and 33% were infected with SIV smm and STLV-I, respectively. While the proportion of female mangabeys (66%) differed significantly (P< 0.01) from the proportion of male mangabeys (42%) infected with SIV smm, the proportions of males and females infected with STLV-I were similar, suggesting independent transmission of the two viruses. Among mangabeys less than 1 year old, none were infected with STLV-I and only five of 27 mangabeys, all of which were at least 6 months old when first tested, were infected with SIV smm. The data document that natural infection of sooty mangabey monkeys with SIV smm or in association with STLV-I infection does not result in increased disease or mortality, and that transmission of both SIV smm and STLV-I appears to occur primarily through sexual activity.