To better understand the physiological importance of acid phosphatase activity we examined the effects of inorganic and organic phosphorus growth sources on enzyme activity and 32P uptake in several ectomycorrhizal fungi. Mycelium of eight isolates from four basidiomycete species demonstrated optimal p-nitrophenyl phosphatase activity at pH 4.5 or 5.0. Acid phosphatase activities varied between strains of Scleroderma citrinum and between the species examined. Interspecific differences in isozyme patterns of whole cell extracts were apparent in native polyacrylamide gels. The isoelectric points of the predominant phosphatases in whole cell extracts were in the pH 5.0 to 5.5 range. Growth of fungi on inositol hexaphosphate versus inorganic P did not affect the isozyme patterns detected by either electrophoretic method. Growth on inositol hexaphosphate affected surface and soluble activities towards p-nitrophenyl phosphate and inositol phosphate to different degrees in species examined. Phytase activity was sufficient to produce a net release of P in all isolates. Growth on inositol hexaphosphate was associated with increased uptake of 32P from inositol polyphosphate in four of five species examined. Acid phosphatase, measured with p-nitrophenyl phosphate, was positively correlated with 32P uptake. Decreased phytase activities measured for inositol hexaphosphate grown mycelium were associated with increased P influx in such mycelium. Both phosphatase activity and 32P uptake were subject to inorganic P inhibition with 32P uptake demonstrating a greater sensitivity. These results provide further evidence for the role of surface acid phosphatases in organic P utilization by ectomycorrhizal fungi. Key words: acid phosphatase, ectomycorrhizal fungi, phytase.