Effects of host age during each of the 5 d of the last host instar were determined by exposing larvae ofHelicoverpa(=Heliothis)zea(Boddie) to newly larviposited maggots of the larval-pupal parasitoidArchytas marmoratus(Townsend). Based on determinations made in host pupae, lower percentages of older hosts were parasitized and lower percentages of maggots from older hosts developed to the second ins tar. Weights of maggots that parasitized older hosts were lower in 1-d-old host pupae. For older hosts, mean length of the larval stage ofA. marmoratusafter host pupation was longer; weights of youngA. marmoratuslarvae revealed that maggots developed very slowly until just before the host pupated. Surviving maggots developing from older hosts used the longer developmental times to overcome the weight differentials, and weights ofA. marmoratuspupae were the same regardless of host age and parasitoid weight at the beginning of the host's pupal stage. These and other data indicate that complex behavior allows theA. marmoratusmaggot to evade host defenses and grow to a size that enables the parasitoid to avoid encapsulation when it finally enters the host hemocele.