Acid phosphatase of the rabbit erythrocyte membrane was confirmed to be oriented to the outer surface of the cell membrane. The enzyme was competitively inhibited by fluoride ion when p-nitrophenylphosphate was the substrate, and this inhibition by fluoride occurred in the presence. or absence of divalent metal ions which were also not required for the enzyme activity. The inhibition by fluoride suggested that it is due to the direct actions of fluoride rather than through the formation of metalofluorophosphate.