The histochemical localization of carboxylic esterase activity was studied for Procyotrema marsupiformis, a parasite of the pancreatic duct of the raccoon. Appropriate substrates and inhibitors were utilized to differentiate simple esterases from cholinesterases. In all tests the pharynx demonstrated the most intense reaction, apparently due to a mixture of nonspecific (simple) esterases and acetylcholinesterase. A strong reaction for acetylcholinesterase activity was obtained in the oral sucker, acetabulum, pseudosuckers and their glandular cells, tegument and subtegumentary cells in the region of the oral sucker, nerve tissue, Laurer's canal, oviduct, descending uterus, crypts of the holdfast, bursa, genital papilla, and excretory pore. Nonspecific esterases were indicated in the subtegumentary cells of the holdfast, the outer tunics of the testes and seminal vesicle, vas deferens, prostatic cells, and within the crypts of the holdfast. Pseudocholinesterase was not demonstrated. Although numerous studies have been made on the histochemical localization of carboxylic esterase activity in trematodes, only a few species have been examined. Notable among these are studies employing Fasciola hepatica adults (Halton, 1967a, b; Krvavica et al., 1967; Sekardi and Ehrlich, 1962; Thorpe, 1967, 1968) and larvae (Panitz and Knapp, 1967) and Schistosoma spp. (Fripp, 1967; Lewert and Hopkins, 1965; Pepler, 1958). Considerable attention has been focused on esterases of strigeoid trematodes as well, particularly in the works of Bogitsh (1966), Erasmus and Ohman (1963), Ohman (1965, 1966a, b), and Lee (1962). Halton (1968) recently investigated esterase activity in the frog lung fluke Haplometra cylindracea and a variety of other trematodes. Halton (1967b) and Davis et al. (1969) conducted a similar study on the digestive tract of Haematoloechus medioplexus. Procyotrema marsupiformis is parasitic in the pancreatic duct of the raccoon. Its elongate holdfast extends from the acetabulum to the posterior end of the forebody and is completely enclosed by the ventral body wall. Host pancreatic duct tissue fills the holdfast cavity. This tissue is highly vascular hemorrhagic and shows no characteristic pancreatic duct tissue com-