The markless trout (iwame, Oncorhynchus iwame) has long been suspected not to be an independent species, but rather a markless mutant form of the red-spotted masu salmon (amago, O. masou ishikawae). Nevertheless, no field study has examined this issue. Here we report a field study on the reproductive ecology of iwame and amago coexisting in the upper stream of the Inabe River. We found that iwame and amago mate sympatrically, simultaneously, and randomly, and observed five cases of heterogeneous oviposition between the two. Our results suggest that no pre-mating reproductive isolation exists between iwame and amago. Because previous studies have shown the absence of post-mating isolation, we propose that iwame and amago are the same species. Iwame should be considered as a markless form of the red-spotted masu salmon, and O. iwame as a junior synonym of O. masou ishikawae.