We examined the phase variation in Tetranychus kanzawai Kishida with regard to density effects and dispersal capacity. The fecundity of females was reduced when they had lived together with senior female adults during the juvenile period was reduced. This negative density effect was not attributed to direct interference by the senior female adults, but to local alternation of the food resource caused by their feeding damage. Moreover, offspring that had developed with the female adults had a higher dispersal capacity than those that developed without female adults. This physiological and behavioral plasticity in response to local density can be considered as a kind of phase variation.