1. Experiments were conducted to determine the percentage of development and incubation rate of eggs of Corophium triaenonyx at various salinities from 5‰ to 40‰. 2. The eggs develop only through a range of salinities from 7.5‰ to 37.5‰, although the adults are able to tolerate a range bebtween 0.6‰ to 59.8‰. A satisfactory percentage of development occurred between 20‰ and 32.5‰, but the optimum range was between 27.5‰ and 30‰ at a constant temperature of 28° C (±0.1° C). Extreme salinities retarded the incubation period, being 18 and 17 days at 35‰ and 10‰, respectively. 3. In the experiments conducted to determine the combined effect of salinity and temperature, it was revealed that the best results were obtained at a combination of 30‰ and 28° C. The upper limiting salinity appeared to be 35‰ at all temperatures and lower limiting salinity is 10‰ between 15° C and 35° C and 15‰ at 9° C. High temperatures at extreme salinities retarded the development.