African wild dogs (Lycaon pictus) are endangered, with only 3,000–5,000 remaining in the wild. It is believed that wild dogs are unusually vulnerable to viral diseases, particularly rabies and canine distemper (CDV). However, canine distemper has been confirmed by laboratory diagnosis in only one free-living wild dog. The 43,000 km² Selous Game Reserve (SGR; Tanzania) holds approximately 900 adult wild dogs. In a study area of 2,600 km², the population maintained high density (≥1 dog/20.5 km²) from 1991 to 1996. The population was stable, varying 18% below and 9% above the mean density over the 6-yr period. Serum samples (n = 22) collected over 3 yr showed that most individuals were exposed to CDV (59%: 95% confidence interval = 43–76% seropositive) and canine parvovirus (68%: 95% CI = 54–81% seropositive), although none were seropositive for rabies (0%: 95% CI = 0–17%). CDV titers were positively related to age, with no seropositive dogs younger than 1.9 yr. At least five of 13 dogs positive for CDV seroconverted during the study Dogs with high CDV titers did not survive better in the years after sampling (mean survival ± SE for those that died = 638 ± 92 days,). Variation in mean litter size was inversely related to CPV exposure in the SGR and elsewhere. Annual mortality rates were low in comparison to other populations for all age classes (pups: 31 ± 8%, n = 127, yearlings: 22 ± 10%, n = 93, adults: 20 ± 6%, n = 235). Annual mortality rates fluctuated little between 1992 and 1996. These data show that wild dog populations, like those of other canids, can remain stable and demographically healthy despite exposure to CDV and CPV.