The relationship between the eugregarine Pyxinia frenzeli and the black carpet beetle, Attagenus megatoma (=Attagenus piceus), was examined under starvation stress. A period of 42 days without access to food or water, during the insect's last larval instar, caused infected individuals to lose weight significantly faster than uninfected controls. Similarly, infected adult females weighed less after this same stress than uninfected individuals. The eugregarine-free control group was obtained from progeny reared from the second larval instar through pupation on a basal dog mealbrewer's yeast medium to which 8% (w/w) sorbic acid had been added. Data indicated that during a short period of complete starvation infected larvae lose weight almost twice as rapidly as their gregarine-free counterparts. Since partial and complete starvation periods are common among A. megatoma in the field, this apparently constant microbial inhabitant of the larval midgut probably aids in maintaining a check on the size of black carpet beetle field populations.