Three species of the varunid crab genus Cyrtograpsus (Brachyura: Grapsoidea) occur in apparent sympatry along temperate coastlines of the southwestern Atlantic. Two of them, C. angulatus Dana, 1851, and C. altimanus Rathbun, 1914, are common in intertidal habitats and are easily separable by their morphology. In contrast, there is very limited knowledge on the ecology and distribution of the third species, C. affinis (Dana, 1851). In the present study, several specimens of C. affinis were collected in the Río de la Plata Estuary between Argentina and Uruguay. The study of these specimens showed that C. affinis tends to be smaller than the other two species of the genus and seems to be restricted to subtidal habitats. However, morphometric comparisons suggest the existence of a continuous gradient rather than discrete differences between specimens of C. affinis and C. altimanus. Morphometric differences were as common among subtidal samples as between subtidal and intertidal samples. Likewise, comparison of mitochondrial DNA sequences (16S rRNA) did not reveal diagnostic differences between the two putative species, suggesting a very close relationship between C. altimanus and C. affinis. Possibly the two forms represent a single species with a high ecological and phenotypic plasticity.