Entomophthora muscae (C.) Fres. can be grown in vitro as protoplasts. Light and electron microscopical studies of the in vitro developed protoplasts have demonstrated the absence of an organized wall over the protoplasmic Con A-positive membrane at all stages of growth. The cytological organization is typical of the Entomophthorales with condensed chromatin in the interphase nuclei and small eccentric metaphase spindles. Long strands of endoplasmic reticulum, microubules and vesicles surrounding the plasmalemma may be involved in maintaining the precise shape of E. muscae protoplast. Starvation of the fungus induces the formation of hyphal bodies after deposition of Con A- and WGA-positive wall material at the plasmalemma surface.