The reproductive interference between Panonychus mori YOKOYAMA and P. citri (MC-GREGOR) was studied in peach orchards, where they coexisted. The abundance of the two species differed greatly among individual peach trees, but the seasonal population trends were similar to each other: populations remained at very low densities from May to July, and thereafter began to increase, reaching peaks in October. Ineffective females (females that produced no female offspring), resulting from the interspecific matings between the two species, were observed in both species on the trees where they coexisted. The degree of reproductive interference varied depending on the ratio of the densitied of the two species on individual trees, and was higher on P. mori by P. citri than vice versa. Furthermore, the proportion of females from overwintered eggs of P. mori collected from trees with a mixture of the two species was very low. These results seem to partially explain why P. mori cannot extend its distribution to lower latitudes where climatic conditions are favorable for the species.