The western Mediterranean stonefly genus Tyrrhenoleuctra traditionally includes 3 species: T. minuta (Spain, North Africa, Balearic Islands), T. tangerina (Spain, North Africa), T. zavattarii (Corsica and Sardinia). Since the traditional morphological characters display great and overlapping variation, allozyme electrophoresis was used to clarify taxonomic and phylogenetic relationships within the genus and to discuss biogeographical implications. The results clearly discriminate at least four entities: the Corso-Sardinian unit, for which the name T. zavattarii can be used; the Balearic population, representing an undescribed species; at least two Iberian peninsular species. However, more data on topotypic populations are needed to define the taxonomic status of the Iberian species (including definition of the currently used names T. minuta and T. tangerina). The presence of Tyrrhenoleuctra in Sardinia and Corsica is likely due to an old vicariance event following separation of the Sardinia-Corsica microplate from the Iberian Peninsula. Calibration of the molecular clock (genetic distances vs. Corso-Sardinian plate split from Iberia) resulted in a very low evolutionary rate (0.008 D/my), lower than those found in taxonomically distant groups (including stoneflies) with similar distribution.