Light and electron microscopical studies were carried out in order to characterize the blood cells of the bivalve mollusc, Scapharca inaequivalvis (Bruguière). Cytochemical techniques were also used to investigate peroxidase, phenol oxidase and lysosomal enzyme activity in the cells. Four types of blood cell were found: the more numerous erythrocytes and three populations of white blood cells. The red blood cells were found to be nucleated and contained dense cytoplasmic granules and various organelles; a well-developed Golgi apparatus, rough endoplasmic reticulum, variable numbers of mitochondria and fine cytoplasmic microtubules were observed. The white blood cells were generally smaller and less frequent. The type 1 cells possessed variable numbers of acidophilic, cytoplasmic granules and showed a high degree of vacuolation. The highly granular cells showed more extensive pseudopodia, perhaps indicative of intense phagocytic activity. The type 2 cells were basophilic and possessed few or no cytoplasmic granules. The type 3 cells were infrequently seen; they were relatively large and packed with large, dense, acidophilic, membrane-bound granules. Peroxidase and arylsulphatase activity was found to be present in the type 1 and 2 white blood cells and in the cytoplasmic granules of the erythrocytes. No phenol oxidase activity was detected.