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Larvae of the land-crab, Cardisoma guanhumi , Latreille. were maintained in 24 different combinations of salinity and temperature from the time of hatching. Survival to the first crab occurred in salinities of 15–45 p.p.t., 25° and 30°C. Duration of the five zoeal and one megalops stages was similar in salinities of 20–40 p.p.t., but at 15 and 45 p.p.t. a greater period of time was required for total development. Mortality of all the larvae at 20°C suggests that temperature plays a more important role in survival and distribution of the larvae of C. guanhumi than salinity. Increments of size in crabs during the first seven post-larval molts were similar in salinities of 5–35 p.p.t., 25°C, but in fresh water increase in size at the time of molting was reduced. Although there was no apparent relationship between frequency of molting and salinities of 5–35 p.p.t., the duration of intermolt was reduced in crabs maintained in fresh water, and survival was also lower. From the present study there is no indication that the morphological and physiological processes that are associated with adaptation of the adult crab to the terrestrial environment are initiated during larval development. Although the adult crabs have successfully penetrated the terrestrial environment, the pelagic larvae are still subject to the numerous ecological variables of the estuarine and marine environments.