The taxonomy of the genus Leuciscus in the Iberian Peninsula has been presented on the basis of morphological analysis. Two species are believed to occur in Portuguese inland waters, L. carolitertii, the northern basins chub, and L. pyrenaicus, which was described for the Tejo and southern basins. Since only slight differences in the meristic and morphometric characters are observed, the distinction between the two species has been made previously mainly on the basis of osteology. In order to investigate the patterns of genetic differentiation between these species, 24 sites were sampled for both species, and examined electrophoretically for the study of variation at 27 presumptive loci. The results pointed to different levels of population differentiation among drainages and support the recognition of the species level for L. pyrenaicus and L. carolitertii. Our results also suggested that the distribution area of L. pyrenuicusis probably restricted to Tejo and to the small drainages near Tejo, while L. carolitertiiseems to have a widespread distribution area, including Lima, Douro, Vouga, Mondego, Guadina and Sado drainages. Moreover, concerning the southern populations, Mira and Arade, the surprisingly high level of differentiation observed at several loci, supports the existence of a further species occurring in these small drainages. The evolutionary aspects related to that differentiation are discussed. 8 1995 The Fishenes Society of the British Isles