Chemical investigation of the secondary metabolites of the marine spongeAplysilla glacialis collected at French Wells, Crooked Island, Bahamas, resulted in the isolation and characterization of four diterpenes, two sterol endoperoxides, and two methylated primary metabolites. Some of these compounds, along with crude extracts of the sponge, were investigated for their ability to deter fish predation, affect the fouling of surfaces, and inhibit the growth of marine microorganisms. The diterpene manoöl (3), cholesterol endoperoxide (4), and the crude nonpolar extract ofA. glacialis were shown to deter feeding by a natural assemblage of fish predators in an in situ assay conducted at French Wells. Pure secondary metabolites and crude extracts ofA. glacialis also were tested in a laboratory fish feeding assay employing the wrasseThalassoma lunare. A mixture of sterol endoperoxides was isolated from the mucus that coats the surface ofA. glacialis and is exuded in large quantities when the sponge is disturbed. These compounds are thereby distributed in a manner in which they can best serve a defensive role for the sponge. An in situ assay was designed to determine the effect that pure secondary metabolites and crude extracts have on the fouling of surfaces. Manoöl (3) and cholesterol endoperoxide (4) were determined to increase the rate of fouling when compared to control surfaces. 1-Methyladenine (5) was identified as an antimicrobial constituent ofA. glacialis that inhibited the growth of four marine bacteria isolated from seawater samples collected at French Wells.