The objectives of this study were to measure larval growth and mortality rates in a butterfly population and to determine how these rates vary locally in space and time. The survivorship and growth rates of 1353 Papilio polyxenes larvae, placed on natural food plants in typical habitats, were followed during both generations of 1973 and 197 4. In each year and generation, cohorts offirstinstar larvae were set out on three commonly used host-plant species and at two elevations. Larvae reaching the fifth instar were returned to the laboratory, where the surviving pupae gave rise to 87 adults. Together with estimates of egg mortality in the field (50%) and the sex ratio of emerging adults (50/50), this result indicates that females must lay at least 60 eggs, on average, to replace themselves once in the next generation.Stage-specific mortality rates (qx) averaged 0.59, 0.33, 0.35, and 0.41 for the first four instars, respectively, approximating a Type II survivorship curve (constant mortality rate). Both the shapes of survivorship curves and the probability of reaching the fifth instar were highly variable from one treatment to another, however, providing a heterogeneous pattern of survivorship in space and time. Survivorship was greater in 1973 than in 1974, probably due to different weather conditions; it was greater at high-elevation than at low-elevation sites; and it was greater on wild carrot, Daucus carota, and on parsnip, Pastinaca sativa, than on poison hemlock, Conium maculatum. Survivorship oflarvae placed artificially did not differ from that of larvae hatching from wild-laid eggs on the same host species at the same time and place.Arthropod predators attacking larvae included spiders as well as hemipterans of the families Nabidae, Reduviidae, Coreidae, and Pentatomidae. Parasitoids emerging from larvae or pupae belonged to the families Phoridae, Tachinidae, Ichneumonidae, and Braconidae. Our results, supplemented by predator-exclusion experiments in 1977, indicate that predation is the major source of larval mortality, small (arthropod) predators having the greatest impact on younger larvae and large (vertebrate) predators the greatest impact on older stages.Larvae grew faster in 1973 than in 1974, probably because of more favorable weather in 1973. Growth was faster at low-than at high-elevation sites and markedly faster on poison hemlock than on wild carrot or parsnip.Unpredictability oflarval survivorship in space and time is likely to favor widely spaced oviposition by P. polyxenes females. Such behavior may also be favored by variability in larval growth rates, since adult emergence times may be crucial to the reproductive success of surviving male progeny.