Pleuropodia of the invaginated type were observed on the first abdominal segment in 8-d-old embryos of Thermobia domestica (Packard). The pleuropodium is formed by a cytoplasmatic internal part and a mushroom-like cavity. The latter is filled with fluid and is composed of a stem protruding through the epidermis and a vesicle-like copula. The arrangement of membrane folds, mitochondria, and lipid drops was observed on electron micrographs (TEM) of pleuropodium cells. The position and structure of these organelles indicates that the cells of this organ perform transport and secretory functions.