The biology ofPodisus serieventrisUhler, and its role in an outbreak of the black-headed budworm,Peronea varianaFernald, in Cape Breton, N.S., are described from studies carried on in 1930 and 1931. There is but one complete generation ofPodisusa year, and adults of both sexes hibernate. The eggs are laid in late June, July and early August, the incubation period ranging between 10 and 15 days. There are five nymphal stages, requiring about 45 days, on the average, for the attainment of the adult condition. In these respects particularly, the life history ofPodisus serieventrisin Cape Breton differs from its life history in Massachusetts, where four nymphal stages and three annual generations and the hibernation of females only, have been reported.The species conforms satisfactorily to Dyar's Law, the average growth ratio of individuals studied in 1931 being about 1.28. The first-stage nymphs feed on unhatched eggs of their own species, and upon the juices of coniferous and deciduous foliage, but were not induced to feed upon small caterpillars. Nymphs were able to complete the first instar on a purely vegetable diet, but died before the second moult when the same diet was continued. Older nymphs, fed for some time on animal food, were not able to attain the adult condition when supplied with plant food alone. This indicates the dependence of the species upon animal food; the food consumption of the various stages is briefly summarized. Evidence is presented which suggests the utilization, byPodisus, of a toxic secretion in overcoming their prey.The rather limited value ofPodisusas a control factor in the outbreak ofPeronea varianain 1930 and 1931 is described. The decline of thePeroneapopulation in 1931 caused a corresponding mortality in thePodisuspopulation, by starvation.