The fatty acid composition of whole egg masses was investigated in 4 marine nudibranchs collected from the southwest of Spain: Polycera aurantiomarginata, Polycera quadrilineata, Berghia columbina and Berghia verrucicornis. The four species are carnivorous. All nudibranchs were characterized by high levels of n-3 polyunsaturated fatty acids (n-3 PUFA), mainly eicosapentaenoic acid (20:5n-3), docosapentaenoic acid (22:5n-3) and docosahexaenoic acid (22:6n-3). Other major fatty acids were the saturated palmitic (16:0) and stearic (18:0) acids, the monounsaturated oleic acid (18:1n-9) and the n-6 PUFA arachidonic (20:4n-6) and docosatetraenoic (22:4n-6) acids. Relative high percentages of the plasmalogen derivatives 16:0DMA and 18:0DMA were also detected. Univariate analysis showed that egg fatty acids from P. aurantiomarginata and P. quadrilineata significantly differed from those of B. columbina and B. verrucicornis. Higher levels of saturated fatty acids and 22:6n-3 and lower percentages of 18:0DMA, 22:4n-6 and 22:5n-3 were found in the spawns of both species of Polycera as compared with both of Berghia. Analysis of the bryozoon Bugula neritina and the sea anemone Sagartia troglodites, main preys of Polycera and Berghia respectively, showed that the differences observed in egg fatty acid composition were likely related to diet. Fatty acids of the fully developed embryos from P. aurantiomarginata and B. columbina, sampled immediately prior to hatching, were analyzed to investigate the dynamics of fatty acids during embryogenesis. The long chain PUFA 20:4n-6, 22:4n-6, 20:5n-3 and 22:5n-3 increased or remained stable during embryogenesis in both species whereas palmitoleic acid (16:1n-7) in B. columbina and 16:0, 17:0, 18:0 and 18:1n-9 in P. aurantiomarginata decreased. The major n-3 PUFA, 22:6n-3, marked decreased in B. columbina but remained unchanged in P. aurantiomarginata. These data indicate that fatty acids play different roles during embryogenesis and that in both nudibranchs the embryo requirements are not the same.