Embryonated eggs of Neopaleorchis catostomi are eaten by the operculate snail, Lithoglyphus virens (family Amnicolidae) in which the miracidium hatches and develops into a delicate, saccate sporocyst which produces a generation of rediae. The latter give rise to chaetomicrocercous cercariae which encyst in the gastropods Ferrissia rivularis, Gyraulus similaris, Physa ampullacea and the sphaeriid clams Pisidium variable and Musculium rykholti, as well as in noninfected L. virens. Metacercariae 50 to 60 days old were fed to young coarsescale suckers, Catostormns macrocheilus, and partially developed worms were recovered 15 and 29 days later. During 1971 and 1972 numerous coarsescale suckers, Catostomus macrocheilus Girard, from the Clearwater River, Nez Perce Co., Idaho, were examined for parasites. Most of the fish contained small spiny trematodes that represent a new genus and species in the family Monorchiidae Odhner, 1911. An investigation of the life cycle was undertaken. MATERIALS AND METHODS Egg capsules of the operculate snail, Lithoglyphus virens (Lea), family Amnicolidae, were collected in late July and early August from the Clearwater River and placed in aerated water for development. As the snails hatched, they were transferred to an aquarium in which an abundant growth of algae coated the glass and the rocks on the bottom of the aquarium. After about a month, this food was augmented with portions of green algal mats from a local pond. These snails feed on desmids, diatoms, and other green algae. They thrived for at least 4 months but none could be maintained to the adult stage. Eggs from gravid trematodes were incubated 2 to 3 weeks in river water which was changed daily and stirred to curb bacterial growth. At least 300 young snails with a shell diameter of about 2 mm were allowed to ingest embryonated eggs. Development of the parasite was followed by dissecting snails at frequent intervals. For use as second intermediate hosts, Ferrissia rivularis Say, Physa ampullacea Gould, and Gyraulus similaris (F. C. Baker) were reared from eggs, using fresh and decaying lettuce as food. The sphaeriid clams, Musculium rykholti (Normand) and Pisidium variable Prime, were collected from nearby Robinson Lake and the Clearwater River, respectively. Fifty of each species were examined for natural infections. One P. variable harbored allocreadiid larvae, several M. rykholti contained Received for publication 27 December 1972. echinostome and/or plagiorchioid metacercariae, and 2 contained larvae of Phyllodistomum staffordi Pearse, 1924. Young coarsescale suckers (6 to 7 inches long), for experimental infections, were obtained from Hangman Creek near Tekoa, Washington, where the parasite and the host mollusk do not occur. Dilute Nile blue and neutral red were used to relax and stain live cercariae and metacercariae. Acetocarmine was used to stain nuclei of larval stages. Oil red O and Sudan black B were used to stain lipids in the redia, cercaria, and metacercaria as in Gomori (1952). Lipid was dissolved with 100% ethanol in controls. The periodic acidSchiff technique (PAS) was used only to demonstrate the cystogenous glands in the cercaria and the cyst membrane of the metacercaria as in McManus (1946), except that oxidation with periodic acid was extended to 10 min. Adults and metacercariae were stained with Gower's carmine and sections with Delafield's hematoxylin. Measurements are in microns unless indicated otherwise. Neopaleorchis gen. n. Body fusiform, spinous. Oral and ventral suckers nearly equal, in anterior third of body. Prepharynx, pharynx, and esophagus present; ceca extend halfway through hindbody. Genital pore submarginal, to left of ventral sucker; cirrus sac to left of ventral sucker; seminal vesicle folded but not bipartite; testes tandem to oblique, postequatorial. Ovary lobed, dextral, pretesticular; seminal receptacle absent. Vitellaria mainly postcecal, contiguous dorsal to uterus and testes (Fig. 3). Excretory vesicle tubular, extending as far as anterior testis. Uterus restricted to hindbody, metraterm present. Eggs small, operculate. Type species: Neopaleorchis catostomi sp. n.