FIG. 1. Top: _Hybopsis gracilis gracilis_, 230.0 mm standard length, one of the largest specimens examined. Missouri River, Carson County-Walworth County line, 3 miles northeast of Mobridge, South Dakota, at mouth of Grand River. Bottom: _Hybopsis gracilis gulonella_, 121.6 mm standard length, the largest specimen examined of this subspecies. Beaver Creek, Fremont County, 10 miles northeast of Florence, Colorado, on Highway 115.] [Illustration: FIG. 2. Regression of head-depth on standard length in _Hybopsis gracilis gracilis_ from the Saskatchewan River, and in _H. g. gulonella_ from Beaver Creek, Arkansas River Drainage (KU 4769).] DISCUSSION _Hybopsis gracilis_ is highly variable in several morphological characteristics, including size and shape of head, body, and fins, and number of scales, vertebrae, and fin-rays. The variations are correlated in a way that indicates the existence of two subspecies. One of these, _H. g. gracilis_, attains large size, and has 1) a slender, streamlined body, 2) a depressed head that is acutely wedge-shaped in profile, 3) strongly falcate fins with the dorsal and pelvic fins originating anteriorly, and 4) many scales, vertebrae, and pectoral fin-rays. The second subspecies, for which _H. g. gulonella_ is the oldest applicable name, is small, and has 1) a deep, chubby body, 2) head convex in dorsal contour (less depressed than in _H. g. gracilis_), 3) fins less falcate than in the latter subspecies, with the dorsal and pelvic fins originating more posteriorly, and 4) fewer scales, vertebrae, and pectoral fin-rays than _H. g. gracilis_. These differences are consistently expressed throughout the size-ranges of the subspecies, and in series collected at the same or nearby localities in several different years. Considerable variability was found in features other than those mentioned above, but individual variation among specimens from the same locality and adjacent localities is so great that none is diagnostic of subspecies. For example, orbital size and length of fins (but not their falcate shape) are variables that have little diagnostic value, although both features seem to vary in clinal fashion, with the higher values in the north.