The major lipid classes, their fatty acid profiles, and the amounts of the lipid-soluble components, vitamin E, vitamin A, and carotenoids, were determined for egg yolks of the Hermann's tortoise (Testudo hermanni boettgeri) with the aim of identifying any features that may potentially impair the adaptation of this endangered species to deteriorations in habitat. Total lipid formed 16% (wt/wt) of the fresh yolk and consisted of (wt/wt) 74.4% triacylglycerol, 18.1% phospholipid, 3.0% cholesteryl ester, and 3.4% free cholesterol. Despite a diet based on green plants, contributing alpha-linolenic acid as the main polyunsaturate, this fatty acid formed only 3.8% of the total mass of fatty acid of the total lipid. The main acyl component of the yolk lipids was the monounsaturated fatty acid, oleic acid, which formed 45.6% of the total. The most striking feature of the yolk composition was the almost complete lack of two nutrients, docosahexaenoic acid and vitamin A, which are essential for the developing embryo. Although it is feasible that the embryo synthesizes docosahexaenoic acid from yolk-derived alpha-linolenic acid and also converts yolk-derived beta-carotene to vitamin A, the yolk is poorly endowed with both these precursors. The stringencies displayed by the yolk composition in this species may limit the flexibility to adapt to changes in the availability of food items when the habitat is threatened. Zoo Biol 20:75-87, 2001. Copyright 2001 Wiley-Liss, Inc.