Acid phosphatase activity was demonstrated in the intact blood nematodes, Dirofilaria immitis and Angiostrongylus cantonensis. Biochemical studies on acid phosphatase, using intact females and whole worm, body-wall and visceral homogenates were undertaken to clarify the characteristics and the distribution of the enzyme. In D. immitis, high acid phosphatase activity was observed at pH 3.8--5.8 and in A. cantonensis, at pH 4.5--6.0. Molybdate, fluoride, copper and zinc ions and L(+)-tartate were effective inhibitors of the enzymes of both parasites while cobalt and magnesium ions and D(-)-tartrate had no noticeable effect on the activity. When the effect of substrate concentration on the phosphatase activity was studied, kinetic curves of Michaelis-Menten type were obtained with the 2 species of intact worms as well as their homogenates. The reproductive organs and body wall of D. immitis showed high total acid phosphatase activity. In A. cantonensis, the majority of the enzyme was localized in the body wall. The activity of intact A. cantonensis expressed as microgram Pi/h/mg dry weight decreased with increase in mean worm weight. The characteristics of the acid phosphatase of the 2 nematodes are compared with those of other parasitic helminths and of acid phosphatase of mammalian origin. The localization of the phosphatase responsible for the hydrolysis of the external substrate has been discussed for D. immitis and A. cantonensis based on results of the kinetics and distribution of the enzyme.