It has been found that the acid and alkaline phosphatases in homogenates of respectively intestine and hypodermis ofAscaris suumhydrolyse sodiump-nitrophenyl phosphate at about twice the rate of sodium β-glycerophosphate. This difference was also observed histochemically. Thus, when sections of intestine were incubated for acid phosphatase withp-nitrophenyl phosphate as substrate, the intensity of staining was about twice as great as that obtained after incubation in β-glycerophosphate. Further, alkaline phosphatase was evident in sections of hypodermis after only 2 hr incubation inp-nitrophenyl phosphate but was not apparent after 10 hr incubation with β-glycerophosphate. Hence biochemical assays and histochemical studies both indicate thatp-nitrophenyl phosphate is a superior substrate to β-glycerophosphate for the visualization of acid and alkaline phosphatases in tiss