664 ciscoes were analyzed from 12 northern Manitoba lakes sampled in 1967 and 1968. In Big Athapap, Coregonus reighardi was distinguished from two sibling species (C, artedii and C. hoyi) by its vertical premaxillae, included lower jaw, terete body and 24 to 36 short gillrakers. Discriminant function analysis separated the sibling species. The three species differed in their lateral line scales, pectoral and pelvic rays, gillrakers, gillraker length, lower jaw, head length, eye diameter, pectoral length and pigmentation, dentition, supraethmoid bone, and growth rate. Body depth of all species, and gillraker number of C. artedii were correlated with maximum lake depth. Correlation and factor analysis confirmed the identifications. Differences between the species from Big Athapap and the Great Lakes and L. Nipigon are probably non-genetic. C. zenithicus C. reighardi may be ecophenotypes of one species, as may C. artedii and C. nigripinnis; C. hoyi and C. kiyi; and johannae and C. alpenae. In Big Athapap, C. reighardi was the most benthic and C. artedii the most pelagic. C. reighardi ate the greatest per cent volume of benthic and bentho-pelagic organisms, C. hoyi of plankton, and C. artedii of surface organisms. All species are autumn spawners; the atypical spanning time of C. hoyi and C. reighardi (compared with the Great Lakes) is associated with their most northerly known occurrence.