In order to determine the distribution and composition of freshwater amphipod species of the Lake District Region of Turkey, detailed samplings were conducted between 1998 and 2001. Among these materials, those from Pinar Pazari and Yukari Gokdere villages (Isparta Province, Turkey) were studied in the present study. Because of having different character combinations, the author believes that they belong to a new species, Gammarus deryae sp. nov. The new species is a typical member of the Gammarus pulex-group having setose third and fourth pereopod and uropod 3 where dense plumose setae exist. Elevated urosomal segments and sharply pointed third epimeral plate are the most distinct characters of the species. In addition, the palp of right maxilla 1 has 1 seta on the outer margin. These encountered characters are commonly observed in the G. pulex-group but the combination of these characters is specific for the newly identified species. All the extremities of the holotype male and allotype female are illustrated and described in detail. The similar and different morphological characters of the new species are compared with those of related species.