The phospholipid fatty acids from the Caribbean sponge Ectyoplasia ferox were studied. The novel fatty acids 25-methyl-5,9-heptacosadienoic (1) and 26-methyl-5,9-heptacosadienoic (2) were identified in 3.4 and 2.0% abundance, respectively, representing the longest set of delta 5,9 iso and anteiso acids yet isolated from a marine sponge. The new acid 10,13-dimethyltetradecanoic (3), the unusual acid 15-methyl-11-hexadecenoic (4) and the also novel acid 9-methyl-11-hexadecenoic (5) were also identified in E. ferox. The principal sterols isolated from E. ferox were 24-ethylcholest-5-en-3 beta-ol (46%) and 24(R)-methylcholesta-5,22-dien-3 beta-ol (14%).