An acid phosphatase from crude culture filtrate of Aspergillus ficuum was purified to homogeneity using three ion exchange chromatographic steps. SDS-PAGE of the purified enzyme gave a single stained band at approximately 68-KDa. The mobility of the native enzyme in gel filtration chromatography, however, indicated that the molecular mass to be about 130-KDa implying the active form to be a dimer. On the basis of a molecular mass of 68-KDa, the molar extinction coefficient of the enzyme at 280 nm was estimated to be 3.4 x 10(5) M-1 cm-1. The isoelectric point of the enzyme, as judged by chromatofocusing, was about 4.0. The purified enzyme is highly stable at 0 degree C. Thermal inactivation studies have indicated that the enzyme is unstable at 70 degrees C. The enzyme, however, exhibited a broad temperature optima with a maximum catalytic activity at 63 degrees C. The Km of the enzyme for p-nitrophenylphosphate is about 270 microM with an estimated turnover number of 2550 per sec. The enzyme is a glycoprotein as evidenced by the positive PAS staining; the sugar composition suggests the presence of N-linked high mannose-oligosaccharides. A partial N-terminal amino acid sequence up to the twenty-third residue was obtained. The enzyme was inhibited competitively by inorganic orthophosphate (Ki = 185 microM) and non-competitively by phosphomycin (Ki = 600 microM).