The fungus, Harpella melusinae, attached to the peritrophic membrane of black-fly larvae, was fixed in glutaraldehyde-acrolein-osmium and in KMnO4. The holdfast does not penetrate the peritrophic membrane, and is comprised of many finger-like outgrowths (“digits”) surrounded by secretions of cementing substance that arise from a specialized basal part of the thallus. Undifferentiated cells contain many organelles, membranes, and lipid droplets commonly found in other phycomycetes, but in addition contain large concentrations of electron-opaque material with vesicles and tubules. The cytoplasm also has crystals in membrane-bound vesicles, myelin-like aggregations of membranes, and large nuclei with associated semi-circular plaques. Septa with small pores like bordered pits, plugged with an electron-opaque material, resemble those of other Harpellales and Linderina (Mucorales). As a spore grows outward, 4 appendages are formed within the generative cell, attached to the base of the spore body, and form a spiral between the wall and plasmalemma. “Forming vescles” transport material from the cytoplasm to the growing appendages, resulting in a periodic structure of alternating dark and light bands. Mature spores have a thick, wavy (in cross section) inner wall and a loose outer wall, and are separated from the generative cell by a septum with a pore.