Some factors which affect the population size ofThyridopteryx ephemeraeformis(Haworth) were studied by inspection of its cases collected from the field in witer. Average fecundity, frequency of reproductive females, percentage females, parasitism, and predation varied significantly among samples from different host plants and among samples from trees ofJuniperus virginianaL., all in the same grove. Percent mortality from unknown causes was significantly dependent on host species. Frequency of reproductive females was density dependent in bagworms feeding on J. virginiana. Groups of bagworms might escape parasites and other causes of mortality by colonizing new host individuals. Females which fed onAcer palmatum atropurpureumNichols did not reproduce. If a 1: 1 sex ratio can be assumed to occur in bagworms at birth, males, on the average, suffered higher mortality than females.