At eleven o'clock on the morning of October 10, 1955, I watched a female Sturmia incompta (Wulp) deposit eggs on a fourth instar larva of Pholus fasciatus Sulzer. The host was smaller and brighter than average and probably was newly molted. The eggs apparently were of the membranous type, with fully developed larvae, and were easily seen when first deposited. Later that same afternoon, after eclosion of the larvae, the choria shrivelled considerably and were easily detached from the host integument. Presumably the parasites immediately began burrowing through the floor of their choria into the host, for the caterpillar thrashed about in an attempt to crush the maggots with its mandibles. The fly also had appeared to annoy the larva, and only her agility enabled her to avoid the larva. On October 16, the host larva went into the soil. Four days later the host, in moribund condition, had developed a putrid odor, and several holes were apparent in its integument. It had not transformed to the pupal stage. The soil was sieved on October 31, and the parasite pupae, grouped together, were brushed lightly with a camel's hair brush to remove dirt before storage. Immediately, the flies began emerging, and expanded normally. The stimulus to emergence may have been provided by the cleaning process, although this had not been the case in numerous other rearings. An interesting variation in this instance was the parasites' emergence from the larval host rather than from the pupal stage. This might be attributed to the eggs having been laid on a young host, or to variations caused by laboratory rearings. Eleven adults were reared, the life cycle from egg to adult requiring 21 days. Normally, the parasites overwinter as small larvae, probably second instar, associated with the tracheae near a spiracle in the host pupa. In October, 1954, numerous Pholus fasciatus larvae were collected on evening primrose (Jussiaea sp.). They pupated shortly thereafter and appeared normal throughout the winter. On March 1, 1955, one of the pupae which had wriggled normally ten days before was examined again. It had split at the junction of the thorax with the abdomen and was devoid of body contents. The parasite puparia, 20 in number, were found grouped near the escape aperture, with their spiracles pointing into the air space surrounding the host pupal case. On March 16, fifteen flies emerged. The remaining puparia were examined several days later and found to contain fully formed dead imagoes.