The investigation carried out on three Mediterranean population of the genusTigriopusshowed that individuals from the populations Livorno and Rijeka are quite similar from the karyological point of view; on the contrary, the chromosome morphology of the population coming from Trapani is completely different. These results support the hypothesis that the North Tyrrhenian and the North Adriatic strains could represent the extremities of a chain of populations belonging to the same species,Tigriopus fulvus,whereas the South Tyrrhenian population could belong to a different species not yet recorded in this area. Further data collected on the North European species,Tigriopus brevicornis,indicate a great karyological difference between this species and the three Mediterranean populations studied. These results confirm the classification which assigns the populations found respectively in the North European and in the Mediterranean areas to different species.