A new species of haemogregarine, Haemogregarina catostomi, is described from the mature erythrocytes of a fresh-water teleost, Catostomus macrocheilus Girard, at Soos Creek, Washington State. The parasite characteristically takes the form of a deep crescent or U, curving around one end of the infected host cell. Only gametocyte stages are present and only one parasite is found per erythrocyte. The intermediate host and means of transmission are unknown. An undescribed haemogregarine, considered to be the first recorded from a teleost restricted to fresh water in the United States, was found within the mature erythrocytes of the largescale sucker, Catostomus macrocheilus Girard. This parasite (Protozoa: Sporozoa: Coccidia: Adeleidae) is a new species which is named after its host as Haemogregarina catostomi. The fish were collected during their spawning migration at Soos Creek, a tributary of the Green River near Auburn, Washington, in March and April 1961. Of 17 suckers examined, 7 (41.2%) were infected; all were sexually mature fish with fork lengths varying between 30 and 45 cm. The examination of young fish (about 6.5 cm), collected later in the spring and summer of 1961, failed to disclose the presence of the parasite. Fantham, Porter, and Richardson (1942), in reporting different haemogregarines from marine fishes of eastern Canada, also described a coccidian from the leucocytes of Quebec brook trout, Salvelinus fontinalis, which they named Leucocytozoon salvelini. This parasite obviously belongs to the genus Haemogregarina (Laird, 1961) and is the first haemogregarine recorded from a fresh-water fish of North America. Other records of haemogregarines relative to this continent were made by Saunders (1954, 1955, and 1958) from marine fishes of Florida and by Laird (1961), who recently described a new species, H. irkalukpiki, from the sea-run arctic char, Salvelinus alpinus, taken from two streams of northern Canada. Received for publication 29 January 1961. *This study was supported, in part, by the National Institute of Allergy and Infectious Diseases, Grant E-3664. Contribution No. 140, College of Fisheries, University of Washington. TECHNIQUE Blood smears were obtained from the caudal artery of fishes by severing the caudal peduncle, after first wiping the area behind the adipose fin free of water and mucus. The films were airdried, fixed in absolute methyl alcohol, and stained with Giemsa. With this stain the cytoplasm of the erythrocytes is a pale orange-red and the nucleus a dark purple. The haemogregarines within the erythrocytes are sharply differentiated by the light blue color of their cytoplasm and the intense crimson of their chromatin granules. Genus Haemogregarina Danilewsky Haemogregarina catostomi sp. n. Host: Catostomus macrocheilus Girard, the large-scale sucker of the Pacific Northwest. Locality: Soos Creek, tributary of the Green River near Auburn, King County, State of Washington. Specific diagnosis: H. catostomi occurs in the cytoplasm of mature erythrocytes. Only gametocyte stages in the blood cell; no intraerythrocytic schizogony. Typical body shape a U or deep crescent, curving inside one end of the host cell without contacting either the nucleus or the boundary of the cell. Mature forms cupped on the inner side of the crescent. Lacking refractive granules, vacuoles and chromatin particles in the cytoplasm, and without polar caps. Average body dimensions (in microns) (20 specimens): Total body length, 14.50 (range 12.08 to 16.48); length of left extremity from the nucleus, 5.21 (3.28 to 7.20); length of right extremity from the nucleus, 5.20 (2.80 to 7.36); length of nucleus, 4.10 (2.32 to 6.24); width of nucleus, 3.37 (1.84 to 4.48); maximum body width, 4.57 (3.52 to 6.32); depth of curve, 8.18 (5.76 to 9.12). Size of normal erythrocyte (in microns) (20 specimens): Length, 15.38 (range 13.44 to 17.28); width, 10.41 (8.96 to 13.60); length of nucleus, 7.52 (6.08 to 8.88); width of nucleus, 4.53 (3.36 to 5.28). Type specimens: Holotypes and paratypes deposited in the United States National Museum No. 59803; other paratypes retained in the author's collection.