Intracellular acid phosphatase activity has been detected in the marine macroalga Ulva rigida. Cytochemical experiments conducted in vegetative cells evidenced the presence of the reaction product adjacent to or along the plasma- lemma, within cytoplasmic vesicles and associated with different vacuolar inclu- sions such as vesicles and multivesicular bodies. This observation leads to infer that acid phosphatases are part of a system through which the orthophosphate anion is obtained from phosphate esters.