Argentation-TLC showed that cholesterol was the only free sterol present in the ES products of 7to 21-day-old worms. Barrett et al. (1970, J. Parasitol. 56: 10041008) using GLC showed that cholesterol comprised 95% of the sterols, cholestane 2.5% and unidentified lipids the other 2.5% in E. revolutum adults. In our study, argentation-TLC of E. revolutum ES sterols showed only cholesterol. The sterol band in our TLC studies did not contain cholestane because this lipid migrates with the solvent front. Burren et al. (1967, Lipids 2: 353-356) showed that Fasciola hepatica released 0.5 to 1.2 mg of lipid/g of fluke/hr. Berger and Fried (1982, loc. cit.) showed that Amblosoma suwaense and Leucochloridiomorpha constantiae released 0.2 to 0.5 mg of sterol/g of fluke/hr. Our results indicate that E. revolutum adults release 0.06 mg of free sterol/g of fluke/hr. Lipids excreted by trematodes can serve as chemoattractants. E. revolutum adults 7 days old, paired significantly within 1 hr (Fried et al., 1980, loc. cit.). In our study, 7-day-old E. revolutum released 15 to 130 ng of free sterols/worm in 1 hr, suggesting that small amounts of free sterol may be available for chemoattraction. This work was supported in part by a Cottrell Science Grant to the junior author from the Research Corporation of New York City.