Acid phosphatase was cytochemically detected at the ultrastructural level in infective and non-infective promastigotes and in amastigotes of the parasitic protozoan Leishmania mexicana amazonensis. Cerium chloride was used as the capture agent of the phosphate liberated during the hydrolysis of the substrate (Na-beta-glycerophosphate). Reaction product, indicative of enzyme activity, was seen in the outer face of the plasma membrane of many, but not all, infective and noninfective promastigote forms. No reaction product was seen in the plasma membrane of amastigote forms. Reaction product was seen in the endoplasmic reticulum, in the Golgi complex, in vesicles located close to the flagellar pocket and in cytoplasmic structures which may represent lysosomes. No reaction product was seen when the substrate was omitted from or sodium fluoride was added to the incubation medium. The possible role played by the acid phosphatase present in the plasma membrane of Leishmania parasites is discussed.