Plasma membranes isolated from the protozoan parasite Leishmania donovani were enriched in acid phosphatase (E.C. 3.1.3.2) activity. Cytochemically, the enzyme was distributed uniformly on the surface of intact cells and was localized on the external face of isolated membranes. Physical characteristics and orientation of the membrane-bound enzyme suggest that the organism is adapted for existence in hydrolytic environments.