Evidence is presented that the level of acid phosphatase is high in the tissue of Clonorchis sinensis. The properties of the enzyme were studied by (1) various chemical substances that inhibit or activate enzyme activity, (2) pH-activity curves, and (3) starch gel electrophoresis. The pH-activity curves revealed three peaks at pH 4.0, 4.5, and 5.5. The enzyme was separated electrophoretically into three fractions, with different Michaelis constants. The results suggest the presence of acid phosphatase isozymes in the parasite. This enzyme was found to be distributed all over the body surface. Alkaline phosphatase was found to be low in the tissue of the parasite. Acid and alkaline phosphatases in parasites have been much investigated. Work published before 1949 on phosphatases was reviewed by von Brand (1952). In recent years, these enzymes have been studied in parasites such as Acanthocephala (23 species; Bullock, 1958), Ancylostoma caninum (Murakami, 1961), Ancylostoma duodenale (Marzullo et al., 1957), Ascaridia galli (Marsh and Kelley, 1959), Ascaris lumbricoides (Thanaka, 1961; Chowdhury et al., 1955b), Cysticercus tenuicollis (Erasmus, 1957a), Dipylidium caninum and Fasciola hepatica (Tarazona Villas, 1958), Haemonchus contortus (nine species of helminths were studied; Marsh and Kelley, 1958), Hymenolepis nana (Gilford, 1957), Moniezia expansa (Erasmus, 1957b; Tarazona Villas, 1958), Schistosoma mansoni (Dusanic, 1959; Robinson, 1961), Taenia pisiformis (Erasmus, 1957b; Tarazona Villas, 1958), Taenia saginata (Chowdhury, 1955), and Wuchereria bancrofti (Chowdhury et al., 1955a). A majority of these studies dealt with histochemical aspects of the distribution of phosphatases in parasites, and some others investigated the pH-activity curve