We estimated population sizes, densities, structure, and the sex ratio of a locally endangered species of mud turtle, Kinosternon scorpioides albogulare, on the Caribbean island of San Andrés, Colombia. We estimated the population size at 4343 turtles on the island in 2002, inhabiting both freshwater ponds and mangroves. Densities for the main populations varied from 77 to 254 turtles/ha. The population structure showed that most of the individuals belong to intermediate size classes (110- to 140-mm carapace length) and a few belong to the smaller or larger size classes (< 110- or > 140-mm carapace length). The sex ratio was significantly different from 1:1 and was skewed toward females, 1 male per 1.97 females. No common use was found for the species among native islanders, although people from the continent living on the island use it for diverse purposes. Some threats were identified, such as mangrove pollution and the presence of the introduced species Tupinambis teguixin.