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Several phosphatases are present in soils. This study was conducted to determine the distribution of phosphomonoesterases (acid and alkaline phosphatases). Results showed that the activities of these enzymes were associated with surface soils and that they decrease with depth. Acid phosphatase was predominat in acid soils, and alkaline phosphatase was predominant in alkaline soils. The activites of these enzymes were significantly correlated with organic C. The log of acid phosphatase activity was significantly but negatively correlated with the pH of 38 samples of 5 soil profiles examined. Regression analysis showed that acid phosphatase activity was significantly but negatively correlated, while alkaline phophatase activity was significantly and positively correlated, with the pH of 11 surface soils studied.Orthophosphate inhibited phosphomonoesterases in soils. At 10 μmole PO43-/g of soil, inhibition of acid phosphatase activity ranged from 21 to 42 percent, and the inhibition of alkaline phosphatase activity ranged from 17 to 51 percent, in 6 surface soils. The inhibition of these enzymes by orthophosphate decreased when the amount of orthophosphate added decreased from 10 μmole to 1 μmole/g of soils. The inhibition by orthophosphate showed competitive kinetics. The results explain the variation of Km values of p-nitrophenyl phosphate for acid and alkaline phosphatases and the observed decrease in phosphatase activity with organic P mineralization in soils.