The nonspecific acid and alkaline phosphatases of Thermoactinomyces vulgaris were found to be optimally active at 65 degrees C and 70 degrees C, respectively, indicating the thermophilic nature of these enzymes in this obligate thermophile. Mg2+, when added in the assay mixture (in the form of MgCl2), increased the specific activities of these enzymes without affecting their respective temperature optima. This divalent cation decreased the Arrhenius energies of activation (E-A) of both acid and alkaline phosphatases, as substantiated by Mg2+-dependent decrease in the slopes of their Arrhenius plots, which were found to be linear. Thus, Mg2+-dependent stimulation of high temperature catalysis of T vulgaris phosphatases appeared to be accomplished by the decrease in their EA values by this divalent cation, and such unique feature of these enzymes might be associated with their evolutionary adaptation in this thermophilic actinomycete to support its growth at elevated temperatures. The catalytic role of Mg2+ in enhancing the phosphatase activities was specified by the fact that this metal ion was able to recover the enzyme activities inhibited by dialysis and EDTA.