The life-cycle of Apatemon (A.) gracilis was completed in the laboratory. The snail host is Lymnaeaperegra (Müller). The cercaria is redescribed from a wide range of material. Metacercariae were found in naturally infected rainbow trout (Salmo gairdneri Richardson), three-spined sticklebacks (Gasterosteus aculeatiis L.) and stone loach (Nemacheilus barbatulus (L.)) from Scotland and in three-spined sticklebacks from Iceland. In trout, most metacercariae were found in the pericardial cavity, in sticklebacks, the eye, and in loach, the body cavity. In infection experiments, cercariae from naturally infected Scottish snails developed in threespined sticklebacks, rainbow trout and brown trout (Salmo trutta L.). Under experimental conditions cercariae did not penetrate stone loach, although this species is naturally infected with A gracilis. The phenomenon of fish host specificity is briefly discussed. Development of the metacercaria is described. Excystation of metacercarial cysts with pepsin and trypsin solutions is unlike that reported for any other digenean; the contents of the cyst appear to be under pressure. In pepsin, layers of the cyst wall peel back from one end. When transferred to trypsin, one pole of the cyst ruptures and the worm is forcibly expelled.