The larvae of the stone crab,Menippe mercenaria (Say) have been reared in 18 different environments—comprising 6 levels of salinity (10, 20, 25, 30, 35 and 40‰) and 3 levels of temperature (20, 25 and 30 C). The results show that both salinity and temperature affect the rate of larval development as well as larval survival. The rate of development was slightly slower in a reduced salinity (20‰) than in high salinities (30–40‰), and very markedly retarded with decreasing temperature from 30 to 20 C. Total mortality was observed in the first zoeal stage in 10‰ at all temperatures. In other experimental salinities (20–40‰) at 20 C, the larvae only developed to the megalopa. Survival to the megalopa at 20 C was less in 20–25‰ than in 30–40‰ and also lower than that observed at 25 and 30 C. Survival to the first crab at 25 and 30C was considerably lower in 20‰ than in higher salinities. The optimum temperature for the larvae ofM. mercenaria would be approximately 30 C, and the optimum salinity in the range of 30–35‰. Under these conditions, the megalopa aas reached on the 14th day after hatching, and the first crab on the 21st day, with total survival in the range of 60–72%. From the relatively high survival and the short development period, it would appear that the stone crab might be a more suitable species than other commercially important crabs for any future large-scale rearing experiments.