Abstract.1Atypical of the parasitoid Hymenoptera, femaleGoniozus nephantidisMuesebeck remain with offspring until they pupate. This behaviour will only have evolved if the consequent reduction in fecundity is outweighed by fitness returns.2G.nephantidiscompetes for hosts with conspecific and non‐conspecific parasitoids. The effectiveness ofG.nephantidisat deterring superparasitism and multiparasitism is tested. Brood survivorships were compared whenG.nephantidisandBracon hebetorSay intrude on hosts with the mother present and absent and with offspring at different developmental stages.3Laying by the intruder reduced brood survivorship. Guarding reduced oviposition on unparasitized hosts by intruding females, and prevented superparasitism of hosts with egg broods. Hosts with larval broods were rarely superparasitized. Multiparasitism was common except on hosts with guarded larval broods, but even here survivorship was reduced.4Competitive asymmetries determined the outcome of contests for possession of host resources.5The costs of remaining seem at least partially offset by the prevention of oviposition by competing parasitoid