Distribution pattern and introgression of two groups of Odontobutis obscura, West Seto and Hikimi Groups, around the Takatsu River were surveyed utilizing six diagnostic allozyme markers. West Seto Group fish were distributed to the middle to lower reaches of the Takatsu River and the upper reaches of the Abu River, while Hikimi Group fish occurred in the upper reaches of the Takatsu River, the Tama River and the Masuda River. West Seto Group population must have invaded the Takatsu River from the Abu River through presumed tributary exchanges between the upper reaches of the Abu River and the Tsuwano River, atributary of the Takatsu River. Slight accidental introgressions were recognized in two localities from Hikimi to West Seto Group populations. However, these two groups were distributed parapatrically and well isolated each other in the Takatsu River basin.