A new ceramide N-docosanoyl-d-erythro-(2S, 3R)-16-methyl-heptadecasphing-4(E)-enine (C22 ceramide) was isolated from the marine sponge Haliclona koremella as an antifouling substance against macroalgae. The structure of this substance was elucidated by spectral means. Antifouling activity of several related compounds was also examined.