Reproductive incompatibility occurred between the Sapporo (43°N) and Tsukuba (36°N) populations of the spider mite, Tetranychus quercivorus. This incompatibility was unidirectional : the Sapporo female was incompatible with the Tsukuba male, which resulted in a low egg hatchability and strongly male-biased sex ratio, whereas the reciprocal cross was comparable to intrapopulation crosses which produced normal progeny with a female-biased sex ratio. To document the geographical extent of incompatibility in local populations, these two populations were crossed with 48 local populations collected from the northern part of Japan. There appeared to be a boundary fro the incompatible area at the Shimokita Peninsula (41°N). Females from the southern populations produced female offspring when crossed with males from either Sapporo or Tsukuba, but few female offspring were obtained from crossings between females from the northern populations and males from Tsukuba. Thus, incompatibility of T. quercivorus is common and extensive among Japanese populations.