The flathead chub, _Hybopsis gracilis_ (Richardson), occurs in the Plains Region of Canada and the United States, in four major drainage systems: Mackenzie River, which discharges into the Arctic Ocean; Saskatchewan River, which discharges into Hudson Bay via Nelson River; and Missouri-Mississippi System and Rio Grande, both draining into the Gulf of Mexico. Each of these systems is occupied in part only. In the Mackenzie Basin, _H. gracilis_ has been reported as far north as Fort Good Hope (Walters, 1955:347). Flathead chubs occur in the Saskatchewan Basin from Alberta eastward to Lake Winnipeg, Manitoba, but have not been found in other streams that flow into Lake Winnipeg (Red River, Brokenhead River and Whitemouth River) nor in Nelson River downstream from Lake Winnipeg. In the Missouri Basin the species occurs more or less continuously from the high plains adjacent to the Rocky Mountains in Montana and Wyoming down the mainstream of the Missouri River to its mouth, and down the mainstream of the Mississippi River as far as Barfield, Arkansas, but not to the Gulf. The species probably attains its greatest abundance in the Missouri Basin, but it is scarce or absent in tributaries north and east of the Missouri mainstream, in the South Platte Basin, and in the central part of the Platte River in Nebraska. The flathead chub is unknown in the Mississippi Basin above the mouth of the Missouri River, and in the Ohio River Basin above its mouth. In the Arkansas River Basin, records are restricted to (1) the headwaters and tributaries of the Arkansas River from eastern Colorado downstream as far as Garden City, Kansas, (2) the Cimarron River at Kenton, Cimarron County, Oklahoma, and (3) the South Canadian River and tributaries from northeastern New Mexico eastward as far as Norman, McClain County, Oklahoma, but rarely there. Thus, the range in the Arkansas Basin seems to consist of three isolated segments. Likewise, isolated populations exist in the Rio Grande System, where flathead chubs are confined to the upper parts of the Rio Grande and Pecos basins, above the confluence of the Rio Grande and Pecos Rivers. Records resulting from introductions have been reported for the Gila River by Koster (1957:62) and from the Snake River, Wyoming, by Simon (1946:72).