Despite the intense global interest in marine natural products all over the world, the natural products chemistry of marine flora and fauna of the western Indian Ocean is still largely under-investigated. Yet, in this part of Indian Ocean, a set of islands scattered along the southeast coast of Africa has been recognized as a significant biodiversity hotspot (Madagascar, Seychelles, Comoros, Mauritius; Reunion, Mayotte and Iles Eparses). In the course of our research activities concentrated on marine invertebrates from Mayotte, Reunion and Madagascar, we discovered thirteen new guanidine metabolites showing noteworthy biological activities including cytotoxicity against KB and in-vitro antiplasmodial activities. In this communication the structures and activities of these new natural products, which might be useful as biological tools and biomedical agents, are described. The relative and absolute configurations of several of the new compounds were determined by chemical transformations, comparisons with analogs or by molecular modeling.