It is reported that the scopurid stonefly Scopura longa exhibits a unique pattern of eclosion that the emergence timing, where females tend to emerge earlier than the males. However, in the closely related species Scopura montana, we observed no sex-based difference in adult emergence timing. Furthermore, we also found a difference in egg maturation (oogenesis) timing between females of these two species. Their distributional areas are not only adjacent, but they even intersect in several areas. In such cases, however, area-based morphological differences have been observed in terms of some characteristics, while no individuals were identified as exhibiting any hybrid, i.e., intermediate characteristics. Furthermore, our DNA analyses also indicated no gene flow between them. Thus, it is considered that some distinct reproductive isolation between these two species is established. In the breeding conditions, however, mating behavior was observed occasionally between them. That is, the difference of the eclosion pattern between these two species may have contributed to the avoidance of interspecific hybridization by a temporal reproductive isolation. Herein, we will discuss the implications of these results in terms of reproductive isolation between these two scopurid species S. longa and S. montana.