We studied the pattern of reproductive isolation between two groups of the subsocial spider mite species, Schizotetranychus miscanthi. S. miscanthi can be distinguished into two groups by differences in male-to-male aggressiveness, one has high and the other has low male-aggression. Habitats of these two groups are parapatric, and are not easily distinguished. We carried out cross experiments between populations taking the geographical factors into consideration. Results indicated that strong reproductive isolation acts on these different groups. Therefore, we concluded that there is little gene flow between the two groups, even when they are distributed in close proximity.