The classification and systematics of the species of the genus Azygia Looss, 1899, especially Azygia angusticauda, are in a state of confusion, and the species interrelationships are unclear. This problem is due in part to the fact that the complete life cycle of A. angusticauda has not been determined and to the exclusive use of variable adult characters in classification. In 1969 and 1970, studies were carried out on A. angusticauda from northern lower Michigan. On experimental feeding, longtail Azygiid cercariae from the prosobranch snail Goniobasis livescens (Menke) developed into fully mature adults of A. angusticauda after 60 days in Lepomis gibbosus. Maturing worms were found in L. macrochirus and Ambloplites rupestris after shorter periods of time. Fully mature adults of a. angusticauda were also obtained from naturally-infected Stizostedion vitreum vitreum, Amia calva, and Perca flavescens; mature and immature worms were obtained from naturally-infected Ambloplites rupestris and Esox lucius, respectively. Paper and gas chromatography and chromosome morphology data are presented which provde biochemical evidence for the identity of the longtail Azygiid cercaria and the adult of A. angusticauda and which clarify the relationships between this species and other members of the family. The relationships between three species of non-Azygiids and members of the Azygiidae were also investigated by chromatographic methods. The adult, cercaria, and germinal sacs of A. angusticauda are redescribed. The two major areas from which this parasite and its hosts have been reported, the biotic associates of this parasite, and the life history and preferred macrohabitats of its snail host have been studied. Hypotheses concerning the natural history and the main and subsidiary hosts of A. angusticauda are presented. Attempts to culture cercarial distomes of this parasite in vitro were unsuccessful. All evidence indicates that A. angusticauda and A. longa are valid species.