Two new β-carboline alkaloids were isolated from the bryozoan Cribricellina cribraria. One of these, 1-vinyl-8-hydroxy-β-carboline, was the major cytotoxic component of the extract, while the other had a novel sulphone structure. Several other known β-carboline alkaloids were also isolated from the extract. To investigate structure-activity relationships in these compounds, several derivatives of the major alkaloid were prepared and a number of 1-substituted β-carboline alkaloids synthesised. Nmr spectroscopic studies of these compounds were carried out and previously published ¹³C nmr data for some of these compounds revised. Homarine was isolated as the major water soluble component of the extract and the sterol composition examined. Known β-carboline alkaloids were isolated from the related bryozoan Margaretta barbata. The known biologically active compounds, girolline and hymenialdisine were isolated from the sponge Axinella sp. 2. As some discrepancies between the experimental and published data on hymenialdisine were noted, an X-ray crystal structure analysis was performed. Oroidin, homarine and taurine were also isolated from the extract and the sterol composition found to consist exclusively of ring-contracted A norstanols. An extract of the sponge Stylopus australis was found to contain a new sterol sulphate and two derivatives of this compound were prepared. The full assignment of the ¹H nmr spectrum of the sulphate was achieved with the aid of COSY, nOe, HETCOR and XCORFE nmr experiments. The glyceryl ether, chimyl alcohol was also isolated from the sponge. An extract of the related sponge, Hymedesmia sp. 1 was examined and rhodoic acid isolated as the major water soluble component, along with homarine. The sterol compositions of Stylopus australis and Hymedesmia species 1 and 2 were examined for comparative purposes. Studies on an extract of the sponge Hymeniacidon hauraki led to the isolation of a new furan fatty acid. The sterol corbisterol and its peroxide were isolated from one sample of the sponge and this led to an examination of infraspecific sterol variation in this species. A screening procedure of biologically active natural product extracts has been developed. The procedure involves examining the chromatographic behaviour of the biological activity of an extract and is designed to detect known biologically active compounds and to determine the best means of handling extracts containing unknown biologically active components.