The population biology of the black land crab, Gecarcinus ruricola (Linnaeus, 1758), was studied in the San Andres Archipelago in the western Caribbean. Investigations were conducted on the islands of San Andres (SA), and Old Providence/Santa Catalina (OP). On both islands the mean size differed little between the sexes, but males reached a larger maximum size by ∼5 mm CW. Crabs of both sexes reached greater mean and maximum sizes on OP than on SA (by ∼10 mm CW): this is tentatively attributed to the higher human population and heavier crab exploitation on the latter. The density of crabs was higher on OP than on SA. In natural ‘forest’ habitats it averaged 2174 crabs Ha⁻¹ in the former, and 804 crabs Ha⁻¹ in the latter. On both islands the densities were higher in watersheds adjacent to the coastlines where breeding females migrate to the sea, and to which returning larvae recruit. The total population estimates were about 3 million crabs on OP, and 800,000 on SA. The lower density and population size on SA can be attributed to a combination of environmental degradation and heavy exploitation. The relative growth patterns showed no unusual features. The species is heterochelic, without preferential handedness. The growth patterns of the chelae resulted in modest levels of both heterochely and sexual dimorphism; this has implications for the pattern of exploitation.