An account is given for the first time of the development of Mutela bourguignati (Ancey) Bourguignat, an African freshwater bivalve of the family Mutelidae. Eggs are shed in large numbers into the inner demibranchs. There they develop into minute larvae similar in some respects to the lasidium larvae of certain South American freshwater mussels, but differing in several ways. Each larva consists of a rounded body covered dorsally and laterally by a thin pellicle and provided anteriorly with two lobes clothed with cilia. Posteriorly, and on the ventral surface, are two sets of minute hooks. Anteriorly is a remarkable elongate, flaccid and colourless tentacle more than seventy times as long as the larva itself. There is no gut, nor can endodermal or mesodermal tissues be recognized as such at this stage. After liberation via the exhalant siphon these larvae settle on the cyprinid fish Barbus altianalis radcliffi and there metamorphose and begin a parasitic phase of development. The larval pellicle folds, and its opposed edges fuse so as completely to enclose the larva. Two tubular outgrowths grow from its anterior end into the superficial tissues of the fish and serve both as organs of attachment and nutrition. In their vicinity skeletal tissues of the host are broken down. The main body of the larva, now designated the haustorial larva, elongates and becomes differentiated into a stalk and a bud. The stalk protrudes from the fish and bears the bud clear of the host. Within the bud the adult bivalve eventually develops. The stalk is traversed by long prolongations of the mantle which run into the haustorial tubes and function as absorptive tissues. The cuticle is not calcified nor is it composed of conchiolin. As differentiation proceeds the definitive mantle, which is the lineal descendant of the larval mantle, begins to secrete first periostracum then calcareous matter to form the rudiments of the valves, and a rudiment of the ligament is also formed. As adult features gradually appear the bud is burst and a young bivalve, little more than 1 mm in length, attached to a long stalk emerges. Independent, particulate feeding commences at this stage. Rupture of both cuticular and mantle elements eventually takes place at the point of juncture of stalk and young bivalve and the latter falls away to begin an independent existence. At the time of release it is capable of active locomotion and is able to produce a slender byssus thread. A detailed, illustrated account of these changes, both external and internal, is given, and the development of certain individual organs is traced. The nature of the cuticle of the haustorial larva, the effect of this larva on its host, and the affinities and probable evolution of the larval stages are discussed, and the importance of taking larval development into account in classification is emphasized.