The azygiid trematode, Proterometra dickermani Anderson, 1962, is shown to complete its life cycle in a single host species, the snail Goniobasis livescens (Menke). No vertebrate hosts have been found in nature, and experiments have produced infections in sunfish (Lepomis gibbosus L. and L. macrochirus Rafinesque) and bass [Micropterus salmoides (Lackp~de)] on rare occasions only. Cercariae reach full maturity within the germinal sac in the snail host. Embryonated eggs are deposited and miracidia hatch in the germinal sacs. Some of the miracidia develop into primary germinal sacs without leaving the individual host; others escape and enter other snails. It is probable that an occasional cercaria leaves the snail and is ingested by a fish. Two complete cycles occur during a year, with hatching of miracidia in early spring and late summer. The extreme progenesis in this cycle indicates a primitive position of the genus Proterometra Horsfall, 1933. Five valid species have been ascribed to the genus Proterometra Horsfall, 1933, of the trematode family Azygiidae Odhner, 1911. The life cycles of four of these have been described: P. macrostoma (Faust, 1918) Horsfall, 1933 by Horsfall (1933, 1934) and by Dickerman (1934, 1945); P. catenaria Smith, 1934 by Smith (1934); P. hodgesiana Smith, 1932 by Smith (1936); and P. sagittaria Dickerman, 1946 by Dickerman (1946). The cycles of these species are similar. The adults usually inhabit the pharynx or esophagus of fishes of the family Centrarchidae. Eggs pass from the host in feces. Hussey (1945) observed hatching of miracidia of P. macrostoma after considerable manipulation of the material. No other instances of free miracidia have been reported, and it has been assumed that hatching normally occurs after ingestion of the eggs by the snail. The snail hosts are of the family Pleuroceridae; species of Goniobasis Lea are involved in each of the cycles, and Pleurocera acuta Rafinesque has been reported as an additional intermediate host for P. macrostoma and P. sagittaria. The germinal sac stages-variously described as sporocysts and rediaeoccur in the vicinity of the rectum. The exact location has been stated to be the "coelom" (Dickerman, 1945), "liver and mantle cavity" (Horsfall, 1933), or "mantle cavity" (Smith, 1934). The cercariae which develop in the second or possibly a third generation of germinal sacs are of the "Mirabilis" group of furcocystocercous cercariae. When they emerge from the snail host, the "distome body" is enclosed within a cavity in the tail. As free-swimming organisms the cercariae appear attractive to fish which avidly ingest them. In the fish the body leaves the tail and assumes the position of an adult in the pharynx, esophagus, or stomach. The cercariae of all four species are more mature than is usual in larval trematodes. EXPERIMENTS AND OBSERVATIONS Anderson (1962) reported the introduction of P. dickermani cercariae taken from infections of the snail, Goniobasis livescens (Menke), into sunfish (Lepomis gibbosus L. and L. macrochirus Rafinesque) with recovery of five specimens after numerous attempts. During addiReceived for publication 23 November 1962. * This investigation was supported by research grant E 3654 from the National Institute of Allergy and Infectious Diseases, National Institutes of Health, Public Health Service. t Contribution from the Department of Biology, New Mexico State University and the Biological Station of The University of Michigan.