This Report is a review of the literature of marine natural product chemistry for 1991 and is the ninth in a series of reviews published in Natural Product Reports. The earlier reports cover the period from 1977 to December 1990. The format for this review is identical to that of the previous reports. The review does not provide a comprehensive coverage of all research involving chemicals from marine organisms but concentrates on reports of novel marine natural products with interesting biological and pharmaceutical properties. Biochemical studies involving marine organisms and reports of primary metabolites are specifically omitted. Research on the biosynthesis of marine natural products has, in the past, been reviewed in detail and is not included in this report. Wherever possible, the biological and pharmacological properties of new marine natural products have been reported but papers detailing the pharmacological studies have been omitted. In the area of synthetic organic chemistry, reports of the total synthesis of marine natural products or close analogues are included but papers dealing with methodology directed towards the synthesis of marine natural products have not been covered. No attempt has been made to review the patent literature or conference abstracts.