One year delay Parasitoid wasps lay female eggs or a high proportion of female eggs in favourable host insects because female wasps require many more resources during their development. Many studies have tested the effects of host physiological status on the sex allocation of parasitoids, but few have attempted to test the effects of host behavioural traits. Cotesia glomerata is a gregarious parasitoid wasp that lays eggs in caterpillars of pierid butterflies. The brood sex ratio in C. glomerata females that attacked aggressive host caterpillars was compared with that in females that attacked less aggressive hosts. The male ratio was higher when C. glomerata attacked aggressive Pieris brassicae caterpillars than when it attacked less aggressive Pieris rapae crucivora caterpillars. However, when C. glomerata females were induced to oviposit in anaesthetized P. brassicae caterpillars, the male ratio in their offspring was significantly lower than when they attacked unanaesthetized caterpillars. C. glomerata was attacked by aggressive host caterpillars during oviposition bouts. It is likely that this aggressive host behaviour disturbed the fertilization process in ovipositing C. glomerata females. These results suggest that a behavioural defence by host caterpillars affects sex allocation in the parasitoid wasp C. glomerata Cotesia glomerata (Braconidae): Parasite; Pieris brassicae (Pieridae): Host; Pieris rapae crucivora (Pieridae): Host. English none