Different forms of the hydriid mussel Alathyria jacksoni occur in riverine habitats of the Murray-Darling river system, south-eastern Australia. Electrophoretic data for three enzymes (glucose phosphate isomerase, malate dehydrogenase and malic enzyme) support the assumption that these are morphological variants of one species. Shells from the Murray in South Australia are elongate-oval, with the dorsal margin extended posteriorly as a blade or ‘wing’, whereas shells from Victoria and New South Wales have a dorsal arch and a postero-ventral inflection. The variation appears to be associated with water velocity. The arched shells have comparatively large, strong adductor muscles; the posterior adductors are elongate and both anterior and posterior adductors are displaced ventrally. These internal modifications are reflected in changing shell shape as size increases. The changes apparently enhance the mussel's ability to maintain an anchorage in a strong current, but lessen the effectiveness of its valve seal. An incidental observation is that the related species A. condola Iredale, known from the Murrumbidgee and Lachlan rivers, may also occur in the middle reaches of the Murray.