The life cycle of Hypsopygia postflava, a moth parasitic on nests of the paper wasp Polistes jokahamae, was studied in Mie, Saitama and Tokyo in 2002 and 2003. The emergence of some overwintered-generation moths was extremely delayed, and some second-generation larvae overwintered without pupation; therefore, it is considered that this nest-parasitic moth typically completes three generations per year, with some completing only one or two generations. The proportion of male adults per nest was 56.4% on average, and ranged from 30.8% to 75.0%, but did not differ significantly with the nests. The head widths of overwintering moth larvae varied greatly between the nests and also within some of the nests. Larvae with a head width of <1.08 mm died during overwintering. Larvae provided with pupae of the paper wasp in glass vessels developed to adults, but those provided only with nest materials or the feces of paper wasps did not.