Pyxinia frenzeli, a eugregarine of the family Actinocephalidae, occupies considerable area of midgut lumen in the larva of the black carpet beetle, Attagenus megatoma (=Attagenus piceus). The only other reports of this protozoan are its original description and a note by Léger and Duboscq which were based on material from a related species of dermestid, Attagenus pellio, and dealt only with intrahost stages.In our laboratory the sporozoites were situated intracellularly between the brush border and the nucleus of the midgut epithelial cells. When P. frenzeli is in the early cephalont stage, its host cell hypertrophies, but is atrophied by the time the cephalont is mature. The insect host larva loses its midgut epithelium with each larval molt so infection of this gregarine must begin anew with ingestion of sporocysts by each instar. Pyxinia frenzeli has never been found parasitizing the midgut of the adult black carpet beetle. However, newly emerged adults may carry in their digestive tracts cephalonts, sporonts, and gametocysts enclosed in a blind sac, which is the intact remnant of the larval midgut.Gametocysts are formed in the midgut of the host most frequently by the union of two but often of three sporonts. Shrinking of the residual cytoplasm within the gametocyst is evident 24 hr after deposition in the host feces, and by the 3rd day spores cover the cytoplasm. About the 4th day after deposition the spores are released in a single chain. Pyxinia frenzeli is not autofluorescent at any stage of its life cycle.The protozoan can be eradicated from the black carpet beetle during the larval stage by incorporating 8% (w/w) sorbic acid into its diet. Aureomycin (0.05%, 0.1%, and 0.5%), Fumidil B (0.05%, 0.5%, 5.0%, and 10.0%) and sorbic acid at concentrations of 0.4%, 1.2%, and 4.0% in the diet were ineffective.Heavy infections of this gregarine do not alter weight of the insect larva, survival through adult stage or length of life cycle. The effects, if any, of this parasite (or commensal) on the black carpet beetle are currently under investigation.