An investigation of the dichioromethane extract of the sponge, Xestospongia testudinaria indicated that the extract was composed of approximately 40% sterols, 30% saturated fatty acids, 10% mono-unsaturated fatty acids and 20% poly-unsaturated acids. The sterol profile was found to vary between two collections of the sponge. In the first collection the major sterol was the C30 compound, xestosterol (4), which had not previously been reported to occur in this species. In the second collection there was a wider distribution of components with cholesterol (2a) being a major constituent and xestosterol being present in a much lower percentage than in the first collection. The poly-unsaturated acid fraction contained an extremely complex mixture. The novel brominated bisacetylenic C18 (47) and brominated C28 (65) acids were found to be the major components. Another six novel brominated acetylenic acids, which were very unstable, were also identified as well as an ester of 4 with 47. The method developed for the separation of the poly-unsaturated acids from the other classes of metabolites and for the isolation of the pure compounds is discussed and their structural elucidation, largely on the basis of NIMR spectroscopy is described. From the hexane extract of Carteriospongia foliascens, two novel bisalkylated norscalarane derivatives (114 and 116) and a bisalkylated scalarane derivative (130) have been isolated. By use of high field NMR and multipulse NMR techniques a complete assignment of the 111 and 13C NMR spectra of 130 has been achieved on 1.5mg of material. This represents the first report of a complete assignment of the 1J4 NMR spectrum of a scalarane derivative. The C-4 stereochemistry of 130 was determined by use of 1H NMR spectroscopic techniques, which gave results in agreement with the previously used 13C NMR method.