This paper deals with the geological, historical, bioclimatic and floristic aspects of the northern sector of the Caribbean—Atlantic Subprovince. By means of phytosociological vegetation samples, we focussed our attention on the edaphohygrophilous Pterocarpus officinalis forests and megaforbic plant communities formed by large helophytes. Our paper presents a new endemic association for Hispaniola, the Roystoneo hispaniolanae—Pterocarpetum officinalis nova. Since this association cannot be included in any of the alliances belonging to the class Chrysobalano-Annonetea glabrae Borhidi & Muñiz in Borhidi, Muñiz & Del—Risco 1979, order Tabebuio-Bucidetalia (Lvov 1967) Borhidi & Del- Risco in Borhidi, Muñiz & Del—Risco 1979, we propose a new alliance, Marcgravio rubrae—Pterocarpion officinalis nova, floristically characterised by Pterocarpus officinalis, Philodendron angustatum, Cecropia schreberiana, Cissus verticillata, Dalbergia berterii, Ficus velutina, Inga vera, Palicourea crocea. All the edaphohigrophilous and occasionally hygrophilous forests found in swampy sites either with fresh or slightly saline water should be included in this endemic alliance.