ABSTRACTHalophytophthora exoproliferais described from decaying mangrove leaves in coastal marine environments. It was discovered wholly independently and nearly simultaneously in Atlantic and Pacific mangrove systems, both near latitude 26°N. Key characteristics that distinguishH. exoproliferafrom other species ofHalophytophthoraare: a) sporangial dehiscence tubes containing clear closure material that rapidly expands outward and immediately dissolves at zoospore release; b) external proliferation of the sporangia; and c) readily produced sexual structures.