To clarify the interspecific relationship between Acronicta carbonaria and Acronicta subornata, we conducted the followings: comparative analysis of their genital structures, rearing experiments, and comparison of their DNA sequences. Although the male genital structures are clearly different between the two species, both species were emerged from the eggs laid by A. carbonaria females. Furthermore, the mitochondrial DNA sequences of the COI region (667 bp) were completely identical between the two species. These results suggest that A. carbonaria and A. subornata are seasonal forms of the same species as indicated by Saito (1996).