Biology of the parasiteCotesia flavipesCameron and rearing procedures appropriate for small-scale laboratory production of the parasite are described. Unfed female parasites had a type I survivorship curve and lived an average of 23.8 h. Parasites accepted and used third- to sixth-instarDiatraea saccharalis(F.) as hosts. The sex ratios and number of parasite progeny were independent of host instar. Increasing parasite/host ratios increased the proportion of hosts parasitized; the increase in proportion parasitized was curvilinear, apparently reaching an upper limit at parasite/host ratios>2:1. Parasites had low attack rates, with a maximum of 1.6 attacks per parasite at a 1:4 parasite/host ratio. Some host larvae exposed to parasites died, whereas others failed to pupate. Exposed larvae that failed to pupate either contained encapsulated parasite larvae or molted to the immaculate, diapausing form. Results suggested that rearing is achieved best by exposing females to hosts as soon after mating as possible, and that the parasite/host ratios used for rearing depend on whether the objective is to maximize the total number of parasites produced or produce the most progeny per adult parasite. Parasite encapsulation and host death are likely caused by the noncoevolved association between the Old World parasite and the New World host.