In quasi-gregarious egg parasitoids, the effects of host age on patch use can be significant because host quality declines with host age; this occurs simultaneously in all hosts in a patch (i.e., egg mass). We assessed such effects in the laboratory using Gryon obesum Masner, an egg parasitoid of Euschistus conspersus Uhler. Host eggs hatched ≈6 d after oviposition; therefore, individual parasitoid females were allowed to oviposit on egg masses (14 eggs/mass) of 0-, 1-, 2-, 3-, 4-, and 5-d-old hosts. We measured body size and developmental time of parasitoid progeny from these hosts. On egg masses of 0-, 2-, 4-, and 5-d-old hosts, we observed ovipositional behavior of parasitoid wasps, and estimated rates of host acceptance, progeny survival, and patch residence time. There was a general decline in host quality with host age. Exposure of wasps to 5-d-old hosts resulted in reduced percentage parasitism, lowered acceptance and ovipositional success per patch, increased patch time, reduced survival and body size of parasitoid progeny, and lengthened developmental time of parasitoid progeny. In many of these hosts, however, wasps left the patch after a short time without any successful oviposition. These results confirm earlier studies showing that successful parasitism of eggs declines with host age, and suggest that such age effects should be incorporated into models of patch use by egg parasitoids. The relevance of host-age effects to augmentative biological control is discussed.