The Scarlet Shiner (Lythrurus fasciolaris) and Redfin Shiner (L. umbratilis) are common small minnow species usually exhibiting a parapatric geographic distribution within the Ohio River basin. Historical collection records suggest several areas of possible syntopy along the periphery of distributional ranges, with suspected hybridization based upon observed intermediate morphology of nuptial males. Nuptial males from nine localities with suspected hybridization were collected in June and July 2004; morphometric, meristic, tuberculation, and coloration data were collected and analyzed to test the hypothesis that these species are hybridizing. Based on univariate and multivariate analyses and qualitative assessment of the morphological data, the Green River, Kentucky River, and Salt River drainages in Kentucky and the Scioto River drainage in Ohio all have populations with evidence of past gene flow. Comparisons of current observations with historical collections show an eastward range expansion of L. umbratilis with replacement of L. fasciolaris. The introgression of morphological characters toward those of L. umbratilis in the Green River, Salt River, and Scioto River drainages imply that hybridization may play a direct role in this range expansion. The novel morphology of specimens from Eagle Creek, Kentucky River drainage suggests that hybridization also has resulted in the development of a static, morphologically distinct hybrid swarm. In each case, environmental conditions seem closely linked with the occurrence and net effects of hybridization between these two species.