Two subspecies of H. gracilis are recognized by us: one northern and eastern, characteristically inhabiting large rivers (H. g. gracilis), and one southern and western, characteristically inhabiting small streams (H. g. gulonella). Other scientific names that have been applied to this fish in the past are listed in the synonymy.

H. g. gulonella is a chubby, deep-bodied fish, whereas H. g. gracilis is long and slender. The head of the creek subspecies is deeper and longer than that of H. g. gracilis, being rounded anteriorly when seen in sideview. The head of the large-river subspecies is acutely wedge-shaped in profile. H. g. gracilis has a larger orbit than H. g. gulonella. Fins of H. g. gracilis are more strongly falcate than those of the other subspecies. H. g. gracilis has a greater number of lateral line scales, pectoral rays and post-Weberian vertebrae than the creek subspecies. The large-river subspecies attains much larger size than does the creek subspecies (Plate 24). Except in areas of intergradation, complete separation of the two subspecies can be made on the basis of lateral line scales, pectoral rays, post-Weberian vertebrae and head-depth. The regressions of head-depth on standard length in H. g. gracilis from the Saskatchewan River (several localities) and in H. g. gulonella from Beaver Creek, Arkansas River Drainage (KU 4769) are shown in Plate 24. Although values for the largest specimens of H. g. gracilis are omitted from Plate 24, the regression remains essentially linear to standard lengths of approximately 250 mm. On the basis of head-depth alone, separation of the two subspecies is possible in specimens larger than 40 mm. Similar results were obtained by using the regression of postorbital length on standard length, and could have been obtained by using other proportional measurements.