The genetic variability of seven populations of Cottus sp. from southern France was analysed using starch-gel electrophoresis. Fourteen enzyme systems were screened revealing seven monomorphic and 12 polymorphic genetic loci. Low within-population allelic variation was observed, with frequent allelic fixations and a maximum of two polymorphic loci per population. By contrast, between-population variation was high, with several private alleles and a unique genotypic pattern for each population. A neighbour-joining tree clustering Nei distances showed three major clades corresponding to locations in the catchment area of the Loire, the Garonne and the Mediterranean. Within the Mediterranean group, the morphologically distinct population of the Lez river, described as C. petiti, was not more differentiated than the other populations on the basis of allozymes.