Environmental factors hypothesized to affect production of the Korean strain of the parasitoid,Cotesia melanoscelaRatzeburg, from larvae of the gypsy moth,Lymantria disparL., were examined to determine the optimum conditions for mass rearing. Maximum parasitoid production occurred with an exposure time of 24 h, a parasitoid : host (P:H) density of 5:50, an oviposition temperature of 28°C, use of first- and second-instar hosts, use of 8-d-old ovipositing females, and with exposure of ovipositing females to a light intensity of 1,452 lux. Neither the presence of males within oviposition cups nor time of day (a.m. or p.m.) significantly influenced the number of progeny produced or their sex ratio (P= 0.05). Among those external factors examined, only age of female parasitoid significantly altered the sex ratio of progeny.