not in Young Development of the solitary hyperparasitoid Eurytoma goidanichi Boucek (Hymenoptera: Eurytomidae), which attacks cocoons of the gregarious parasitoid Cotesia glomerata L. (Hymenoptera: Braconidae), was investigated. Within one day after being laid in cocoons of C glomerata, eggs of E. goidanichi were transplanted into healthy host cocoons of various ages. In the laboratory, hyperparasitoid and host development within cocoons was observed through slits in the cocoons. The survival rate of eggs transplanted into young (0-1-day-old or 2-3-day-old) cocoons reached 80% or more; that of eggs transplanted into 4-5-day-old cocoons was 70%. However, eggs transplanted into old (6-7-day-old) cocoons failed to develop, and all the hosts of this age emerged as adults. At 20[degree]C, the mean egg duration of E. goidanichi was 2.8 days. Larvae moulted four times and pupated 8.7 days after hatching, a few days after reaching their maximum size. Mean adult ecdysis occurred 8.9 days after! pupation. The pupae of those E. goidanichi which had developed in 2-3-day-old or 4-5-day-old cocoons were smaller than pupae of wasps that had developed in 0-1-day-old cocoons. Hosts continued development while being devoured by E. goidanichi larvae, but no developmental progress was observed after removal of hyperparasitoid eggs from naturally parasitized hosts. Apparently, E. goidanichi females halt host development at the time of parasitization and are thereby able to improve their utilization of host cocoons. Pieris rapae crucivora (Pieridae): Host; Cotesia glomerata (Braconidae): Host, Parasite; Eurytoma goidanichi (Eurytomidae): Parasite. Cotesia glomerata: Development, host parasite relationships, hyperparasitism, Hymenopteran parasites, Eurytoma goidanichi, Lepidopteran hosts, Pieris rapae crucivora, Parasite and hymenopteran hyperparasite development; Eurytoma goidanichi: Development, host parasite relationships, hyperparasitism, Hymenopteran hosts, Cotesia glomerata, Survival; Pieris rapae crucivora: Hymenopteran parasites, Cotesia glomerata, Parasite and hymenopteran parasite development. Arthropods; Hymenopterans; Insects; Invertebrates; Lepidopterans. none