Three populations (one from a river and two from lakes) of Salaria fluviatilis, the only exclusively freshwater representative of the Blenniidae, showed significant differences in reproductive behaviour and morphology. Breeding males and females were significantly larger at maturity in the river than the two lake populations. The two lake populations, however, showed the greatest degree of morphological difference, the river population being intermediate. The mating system of each population could be described as resourceβbased and promiscuous with parasitic βsneakerβ males that released sperm in the nests of other males. During spawning, males from the river population released sperm significantly more often than the lake populations. This was paralleled by a greater investment in sperm as measured by relative testis mass in the river population. This was interpreted as the need to counteract the loss of sperm during fertilization as a result of the strong flow in the river. Thus some of the patterns of trait variance fitted predictions of adaptations. Other traits, however, varied randomly across populations suggesting change through genetic drift.