High-performance thin-layer chromatography was used to analyze the neutral lipids in the rediae, cercariae, and encysted metacercariae of the paramphistomid trematode Zygocotyle lunata. Visual observations of the chromatograms showed that the most abundant lipid fractions were free sterols and free fatty acids in all larval stages and triacylglycerols in the metacercariae and rediae. The weight of free sterols (x̄ ± SE) was 120 ± 20 ng/cercaria, 56 ± 3.8 ng/redia, and 5.9 ± 1.5 ng/encysted metacercaria; the weight of triacylglycerols was 13 ± 0.88 ng/encysted metacercaria, 6.3 ± 0.063 ng/redia, and was not detectable in the cercaria; the weight of free fatty acids was 160 ± 17 ng/cercaria, 76 ± 9.1 ng/redia, and 4.2 ± 0.46 ng/encysted metacercaria. Oil red O staining of whole larvae showed the presence of neutral lipids in the rediae but not in the cercariae or encysted metacercariae. A dramatic reduction was seen in the quantity of free sterols and free fatty acids in the encysted metacercariae as compared with the cercariae, suggesting that these neutral lipids are used in some way during the transformation from cercaria to metacercaria.