The adult, egg, and larval stages of P. palitans are described, and the adult, male genitalia, ovaries (and associated structures), egg, first-, second-, and third-instar larvae and head structures are illustrated. Unmated females of P. palitans produce only males, mated females produce progeny in a sex ratio of 1:1, but multimated males supply so little sperm at each mating that the sex ratio may go as high as 58:1 in favor of males. The parasite oviposits in aphids of any stage, but apparently prefers the third and fourth instars. Longevity of adult female parasites at constant temperatures, with honey for food, was 10 days at 28° F., 48 days at 50° F., and 1 day at 80°, 90°, and 100° F. Longevity of adult males, with honey, was 7 days at 28° F., 56 days at 50° F., and 1 day at 80°, 90°, and 100° F. Longevity was greatly reduced when honey was withheld. Superparasitism occurs only rarely, since the female is apparently able to detect a stung host. Discrimination of a parasitized host is apparently due to some chemical response by the parasite's antennae rather than to any tarsal or ovipositional response. The egg in the host increases volumetrically by 634 times during its 69 to 71 hours of development. The first-instar larval stage lasts about 25 to 27 hours, with the larva feeding almost exclusively on the cells of the trophamnion. The secondinstar larval stage lasts about 22 to 24 hours, with the larva feeding as in the first instar. The third-instar larval stage in its internal phase lasts about 18 to 20 hours, and during this period all aphid contents are consumed, the parasitized aphid living until a few minutes before the parasite larva emerges. The third-instar larval stage in its external phase lasts about 24 to 26 hours, and during this period the characteristic cocoon is spun underneath the dead aphid host. The five steps of cocoon formation are given, together with comments on the four forms of diapause and nondiapause cocoons. The pupal period lasts about 94 to 98 hours, and some 15 or more meconial pellets are passed about 12 to 19 hours before pupation. A complete life cycle at 70° F. requires about 285 hours, or nearly 12 days. There are probably 15 generations a year in the field. Aphid defenses against parasitization include the production of a copious supply of honeydew, which often traps or hinders the parasite, and the aphid's habit of jumping out of reach of the parasite. The parasite partly overcomes the aphid's saltatorial habit by placing its front legs on top of the aphid during parasite oviposition. A facultative diapause exists and lasts for about 140 days in the field during the winter months; the effect of this diapause on the subsequent parasitization of the aphid is noted; various factors which may limit the effectiveness of Praon palitans are discussed, and the present distribution, hosts, and hyperparasites of P. palitans are noted.