An extracellular (conidial) and an intracellular (mycelial) alkaline phosphatase from the thermophilic fungus Scytalidium thermophilum were purified by DEAE-cellulose and Concanavalin A-Sepharose chromatography. These enzymes showed allosteric behavior either in the presence or absence of MgCl2, BaCl2, CuCl2, and ZnCl2. All of these ions increased the maximal velocity of both enzymes. The molecular masses of the conidial and mycelial enzymes, estimated by gel filtration, were 162 and 132 kDa, respectively. Both proteins migrated on SDS-PAGE as a single polypeptide of 63 and 58.5 kDa, respectively, suggesting that these enzymes were dimers of identical subunits. The best substrate for the conidial and mycelial phosphatases was p-nitrophenylphosphate, but,beta -glycerophosphate and other phosphorylated compounds also served as substrates. The optimum pH for the conidial and mycelial alkaline phosphatases was 10.0 and 9.5 in the presence of AMPOL buffer, and their carbohydrate contents were about 54% and 63%, respectively. The optimum temperature was 70-75 degreesC for both activities. The enzymes were fully stable up to 1 h at 60 degreesC. These and other properties suggested that the alkaline phosphatases of S. thermophilum might be suitable for biotechnological applications.