Aspergillus saitoi produced a several forms of acid phosphatases. Fraction III-A, the largest fraction in them, was purified and proved to be homogenious by ultracentrifugal analysis and polyacrylamide disc electrophoresis. The molecular weight of the enzyme was 12.2×104, and the isoelectric point was pH 4.4. It was a typical acid phosphatase having the activity in only an acidic range. Optimum pH was 3.0. Other properties of this enzyme were also examined. Other four fractions were partially purified and their properties were examined.