The life cycle of Kurosaia jiju, a mite phoretic on the mason wasp Anterhynchium flavomarginatum micado, is described based on the results of field observations and artificial rearing of the host. One to two deutonymphs of the mite, on average, disembarked from a female wasp and migrated into a host cell while the host was laying an egg or provisioning the cell with prey. The deutonymphs quickly molted to tritonymphs and then to female adults, which copulated with their single sons (small-type males) that they produced ovoviviparously and laid a mean of ≈300 eggs for about 1 wk. Tritonymphs and adults fed on hemolymph of prey provisioned by the wasp and facultatively on host larvae, whereas larvae and protonymphs scavenged various organic debris, including prey feces. Eggs of the mite hatched when nonoverwintering prepupal host turned into pupae, and then larval mites molted to protonymphs on the host pupae. Protonymphs became deutonymphs 12–24 h before host eclosion, and they attached to specific parts of adult host surface. On overwintering hosts, deutonymphs made dense aggregations composed of 50–300 individuals on the ventral surface of the thorax of prepupal hosts during winter. Venereal transmission of deutonymphal mites between host sexes was common. The mite gave no apparent detrimental or beneficial effects on fertility, immature survival, or development of the host. The biology of K. jiju is compared with related mites from the viewpoint of parasite–host interaction are discussed.