Unidirectional incompatibility has been known in the spider mite, Tetranychus quercivorus Ehara et Gotoh. In a cross between females from Tsukuba (36°N) and males from Sapporo (43°N), most eggs obtained hatched and reached adulthood with a female-biased sex ratio, whereas in the reciprocal cross, almost all fertilized eggs died and an extremely male-biased sex ratio occurred. Embryonic development was examined to determine the lethal stage of hybrid eggs obtained from the incompatible cross (Sapporo 9 x Tsukubad~). Development of normal embryos was divided into 18 stages, which were described and illustrated. In the incompatible cross, most embryos attained the eye-spot stage without showing any abnormality, but the majority of them stopped their growth then and died at this stage.