1. 1. Phospholipid fatty acid compositions of whole animals from all life stages of the gypsy moth Lymantria dispar and the fatty acid compositions of total lipids of their larval culture medium were determined. 2. 2. The major components of the larval diet and the insect tissues were the C16 and C18 saturated and unsaturated fatty acids that are expected in plant and insect lipids. 3. 3. The fatty acid profiles of the insects and of their larval diet were different. 4. 4. There were small differences in fatty acid profiles of phospholipids and triacylglycerols from different life stages of the gypsy moths, and substantial differences between phospholipids and triacylglycerols within life stages. 5. 5. Certain C18 and C20 polyunsaturated fatty acids (PUFAs) that were not present in the diets were detected in the insect tissue phospholipids. 6. 6. Biosynthesis of certain PUFAs from 18:2n-6 and from 18:3n-3 is inferred from these observations.