Genetic differences within species of tilapias were investigated. These differences are, for instance, expressed in the molecular composition of the erythrocyte membrane.Specimens of Oreochromis uureus (STEINDACHNER 1864) and 0. niloticus (LINNB 1766) were immunized with alloantigenic erythrocytes. Results of agglutination assays with the obtained antisera indicated the occurrence of a minimum of five and seven isoantigens in the investigated groups of 0. aureus and 0. niloticus, respectively. Comparative tests did not reveal an identity of any of the 0. aureus and 0. niloticus isoanti ens.Apart from these ceflular blood group structures, the existence of soluble blood group substances in the plasma of the two investigated species was indicated by WESTERN blot tests.