Three new sesquiterpene carbonimide dichlorides have been isolated from a marine sponge of the genus Axinyssa, and a guaiane-type sesquiterpene peroxide from another Axinyssa sponge. A new isocyanosesquiterpene alcohol was obtained from the nudibranch Phyllidia pustulosa. Their structures were determined on the basis of 2D NMR data. These compounds showed potent antifouling activities against larvae of the barnacle Balanus amphitrite.