none The reproductive response of the gregarious parasite, Glyptapanteles flavicoxis (Marsh), to instars 2-5 of Lymantria dispar (L.) was examined for five host densities (1, 2, 4, 8, and 16 hosts per d). Fecundity was the same at all host densities and instars, but was positively correlated with female longevity. Production of progeny was generally highest on the day females emerged and decreased as the parasites aged. The most hosts parasitized during a 24-h period for any parasite was 12, with an average of 3.7 hosts for parasites exposed to 16 hosts per d. The parasitization rate of G. flavicoxis exhibited a type 2 functional response, such that total numbers of hosts parasitized increased, with those densities, but the percentage declined. Females attacked greater numbers of early (second-third) instars than late (fourth-fifth) instars, but the number of progeny per parasitized host increased with host instar. Conversely, fewer progeny were produced per parasitized host as host density increased. Host density did not have any effect on sex ratio (percentage of females), but the sex ratio in progeny of females ovipositing in late instars was higher than in those ovipositing in early instars.