There is evidence of high mobility on the part of both subspecies, based on irregularity of their occurrence in certain localities. Many collections have been made in the Cimarron River in the vicinity of Kenton, Oklahoma, from 1925 to the present, but only one of these (in 1957) contained flathead chubs. Bait dealers who seine the South Canadian River in Dewey County, Oklahoma, have taken flathead chubs in abundance in some seasons, but not at all in others. Seasonal variation in abundance in the lower Vermillion River, South Dakota (Underhill, 1959:100) has been cited, and the number collected in the lower Kansas River near Lawrence has varied similarly. Many rivers occupied by _H. g. gulonella_ (and by intergrades) are intermittent, and in some years their sand-filled channels become wholly dry for many miles. These factors probably promote mixing of the two subspecies, and may account, over long periods of time, for the wide dispersal of _H. g. gulonella_ in the Missouri Basin. Flathead chubs are known from Pleistocene beds at Doby Springs, Oklahoma (the Doby Springs local fauna) (Smith, 1958:177). Drainage connections between the Arkansas, Kansas and Platte river systems existed in Kansan and Nebraskan times (Frye and Leonard, 1952:189-190). Populations that have subsequently become isolated in those rivers could be accounted for in this way. Flathead chubs could have entered the Rio Grande-Pecos system by stream-capture from the Arkansas System, in northeastern New Mexico or southern Colorado. _H. g. gracilis_ undoubtedly entered the Saskatchewan and Mackenzie basins from the upper Missouri Basin, following glacial retreat (Walters, 1955:347). LITERATURE CITED AGASSIZ, L. 1854. Notice of a collection of fishes from the southern bend of the Tennessee River, in the state of Alabama. Amer. Jour. Sci. and Arts, 17(50):297-308 and 353-365. BAILEY, REEVE M. 1951. A check-list of the fishes of Iowa, with keys for identification. _In_ Iowa fish and fishing, by James R. Harlan and Everett B. Speaker. 1st edition. Iowa Cons. Comm. 185-237. 1956. A check-list of the fishes of Iowa, with keys for identification. _In_ Iowa fish and fishing, by James R. Harlan and Everett B. Speaker. 3rd edition. Iowa Cons. Comm. 325-377. BECKMAN, WILLIAM C. 1952. Guide to the fishes of Colorado. Univ. Colorado Mus., 11:1-110. BRYANT, FRANCIS T. 1858. Report of Francis T. Bryant to Col. J. J. Abert, Chief, Corps Top. Engs., USA. _In_ Executive Documents of the Senate of the U. S., First Session, 35th Congress, and Special Session of 1858. 3:455-481. CHURCHILL, EDWARD P., and WILLIAM H. OVER. 1933. Fishes of South Dakota. South Dakota Dept. Game and Fish. 1-87. CLEARY, ROBERT E. 1956. The distribution of the fishes of Iowa. _In_ Iowa fish and fishing, by James R. Harlan and Everett B. Speaker. 3rd edition. Iowa Cons. Comm. 267-324.