Spalangia endius Walker, a parasitoid wasp, parasitizes both young and old Musca domestica L. pupae, but parasitization site differed with host age. With young hosts, a mother’s first drill attempt was about equally likely to be on either half of the host; and host half did not affect the number of drill attempts, the proportion of those attempts that were successful, or the duration of the first successfully completed drill. In contrast, with old hosts, mothers tended to attempt drilling sooner and more often on the posterior versus anterior half of hosts; and a greater proportion of drills were successful on the posterior half. Offspring head width did not differ significantly between offspring oviposited on the posterior versus anterior half of hosts, regardless of host age. Once adult, most offspring chewed out through the anterior half of the host, regardless of host age.