In order to find characters in Millepora species with a higher diagnostic value than the growth form (on which species delimitation is hitherto based) a comparison is made of the surface structure, microstructure of the skeleton, structure of the ampullae coverings, size of the gastropores and dactylopores, and pore density in specimens of the four species occurring in the Caribbean region: Millepora alcicornis, M. complanata, M. squarrosa, and M. striata. While most characters are of little taxonomic value, pore sizes and densities are surprisingly useful. The species are diagnosed and some biogeographical remarks are given.