Abstract 1. 1. The fatty acid composition of the whole plasma and erythrocytes was determined in four Atlantic bottlenosed dolphins. 2. 2. The plasma contained 27% saturated, 46% monounsaturated and 27% polyunsaturated fatty acids. 3. 3. The red cells contained 33% saturated, 33% monounsaturated and 33% polyunsaturated fatty acids. 4. 4. Arachidonic acid was a major polyunsaturated fatty acid in the erythrocytes (13·3%) but a less important component of plasma (2·5%). 5. 5. The iso, anteiso and multibranched chain fatty acids were minor but significant components of the fatty acid profile of plasma but were present at markedly lower levels, or not detected at all, in the red cell. 6. 6. The plasma fatty acids, which in general reflect the dietary fat intake of the dolphin, exhibited a typical marine fatty acid profile. 7. 7. The erythrocyte fatty acids, being mainly synthesized endogenously, are similar in composition to those of terrestrial mammals except for lesser amounts of 18-carbon unsaturated fatty acids.