In a series of experiments with G. pulex, G, duebeni celticus and G. duebeni duebeni populations, pure or mixed, their tolerances for several temperature/salinity conditions have been determined. Tested were 100%, 10%, 1% and 0.1% seawater in combination with temperatures of 5°, 10° or 15° C. In general all species preferred the 5°C conditions above those with a higher temperature, and the middle salinity range (1% for G. pulex, 1% and 10% for G. d. celticus and G. d. duebeni) above the lower or higher salinities. In nearly all situations tested G. pulex had a lower survival rate than G. d. celticus and G. d. duebeni. Notwithstanding the sometimes unfavourable conditions for G. pulex, this species can compete successfully with G. d. celticus under most of the experimental conditions, but the interspecific relations between G. pulex and G. d. duebeni end in the competitive exclusion of the first species. These results support two theories: (1) G. pulex is able to compete successfully with G. d. celticus and to expel the latter from its niche, and (2) G. d. duebeni is able to invade freshwater and to establish inland populations. The differences demonstrated between G. d. celticus and G. d. duebeni are an argument for a different development and different capabilities of both subspecies. Sterile interspecific matings between G. pulex and both subspecies of G. duebeni are temperature dependent and do not play a major role in the interspecific relations connected with competition.