Scanning electron microscopical studies of acrylic corrosion casts of the blood vascular system of the gills of Pholas dactylus and of critical-point dried preparations of whole gills, supplemented by histological preparations and observations on whole animals, show that the blood vascular system has two interconnecting pathways, one frontal and one abfrontal, both of which arise and terminate in the primary vessels of the gills. In the abfrontal pathway both afferent and efferent primary vessels give rise to secondary vessels at right angles to them. There is no regular alternation of afferent and efferent secondary vessels and both occur in the ascending and descending lamellae. Tertiary vessels, circumferential to the interlamellar spaces, link the secondary vessels. In the filamentary frontal pathway blood is conveyed to or from the primary vessels via perforations in their ventral walls which open into labyrinthic blood spaces; the filamentary frontal channels of descending lamellae empty into these spaces whereas those of ascending lamellae are filled from the spaces. The two systems are joined by many short, D-shaped spaces within the filaments, which connect the filamentary frontal channels to the circumferential, tertiary vessels. Movement of blood in the gills is more oscillatory than unidirectional. Factors that might affect the flow of blood in the two systems and in different parts of the gills are considered.