The influence of host pupal age on host preference and host suitability in Brachymeria lasus (Walker) was examined. Rice armyworm, Mythimna separata (Walker), pupae of different ages, i.e., 0–1, 2–3, 4–5, 6–7 and 8–9 d old were used as hosts. The females accepted the host pupae equally for oviposition among 5 age groups, suggesting that female B. lasus does not discriminate hosts of different ages. However, the rate of progeny emergence was lower in old (6 to 9 d old) hosts. The adult parasitoid progeny produced from young (0 to 5 d old) hosts were heavier in body weight than those from old hosts. The developmental time of the immature stages of progeny was not significantly affected by host pupal age. Adult progeny with heavier body weight emerged earlier than lighter ones when reared on 0- to 5-d-old hosts. However, there was no correlation between the progeny body weight and developmental time of the immature stages when reared on 6- to 9-d-old hosts. Most of the progeny were females and no effect of host age was found on progeny sex ratio, which may be due to the low oviposition rate they experienced.