A population of cooperatively breeding, group-living splendid wrens was tested with a mounted parasitic cuckoo. At all nests with incubated eggs or nestlings, wrens attacked the cuckoo. The timing and intensity of attacks was independent of the nest day and of the age and breeding experience of the wrens. The breeding female usually spotted and attacked the cuckoo first. Her mate and the nonbreeding helpers responded to her call and mobbed and attacked the cuckoo. Response was no quicker in groups with nonbreeding auxiliaries than in single pairs. Discovery time was independent of the number of birds in a group and depended on the movements of the breeding female. Most wrens fed the young and mobbed the cuckoo. When a wren did not attack, it usually was caring for the young of another breeding female or an earlier brood. Variance in helping behavior was not closely associated with variance in the genetic relationship between helper and the breeding female or the young beneficiaries of mobbing. Use of a common territory, attendance at a nest, feeding the young, and mobbing and hitting a cuckoo were all associated cooperative activities. The main limitation of cooperative behavior in defense against the cuckoo is the same as the observed constraint on care of the young during the prolonged period of parental feeding-a conflict of interest among breeding females for care of their own young.