In a field cage, the responses of females of the polyphagous ichneumonid ectoparasitoidExeristes roborator(F.) to a natural host and microhabitat were reduced by prior experience with the same host in an artificial microhabitat. Learning of the artificial microhabitat probably interfered with females' normal responses to the natural system. Parasitoids that had previously attacked hosts in the artificial microhabitat contacted the natural system in fewer numbers and performed host-seeking and host-attacking activities on it with lower intensity than control females. They also killed fewer host larvae than controls. Thus, rearing procedures that permit parasitoids produced for release as biological control agents to gain experience with nontarget hosts and microhabitats could impair these insects' effectiveness against target pests. Prior experience with the natural host and microhabitat did not increase females' responses to them. This result could reflect the way in which learning functions forE. roboratar. Learning might allow the parasitoid to use unusual or less preferred hosts when its normal hosts are rare but not to enhance its responses to hosts for which it has a strong innate preference. Learning may have causedE. roboratorto respond almost exclusively to the natural host and microhabitat when it was familiar with them. Fidelity to a target system caused by prior exposure to it might enhance a biological control agents' performance over a longer term than was examined here.