The fatty acids and nonsaponifiable lipids of Eimeria tenella oocysts were analyzed by gas liquid chromatography and combined gas liquid chromatography-mass spectrometry. The fatty acids detected were identified as C14:0, C16:0, C16:1, C18:0, C18:1, and C18:2. Though the wt of the fatty acid fraction decreased during sporulation from 91 mug per 10(6) oocysts to 47 mug per 10(6) oocysts, the relative amounts of these fatty acids did not change appreciably. The nonsaponifiable lipids of E. tenella consisted of cholesterol and unbranched primary alcohols of 22, 24, 26, 28, 30, and 32 carbons. Mass fragmentography demonstrated that each species of alcohol consisted of saturated and monounsaturated derivatives. Trimethylsilyl ethers of fatty alcohols were found to offer several important advantages over free alcohols for mass spectrometric characterization. Before sporulation, most fatty alcohols were in the oocyst wall. During sporulation, the wt of the nonsaponifiable lipids increased from 16 mug per 10(6) oocysts to 44 mug per 10(6) oocysts due largely to synthesis of C24 and C26 alcohols. The newly synthesized fatty alcohols were not deposited in the oocyst wall.