Atlantic bottlenose dolphins housed in open ocean enclosures in San Diego Bay, California, developed nonpathogenic skin films consisting of a single diatom species belonging to the epizoic, monoraphid genus Epipellis R.W. Holmes. This diatom differs from the single known representative of this genus, E. oiketis R.W. Holmes, by its denser striae and marginal alveoli on both the raphe and araphid valves, a greater number of areolae rows on the marginal rim of the raphe valve, and the presence of an embossment of the external valve surface distal to the terminal raphe ends. Consequently, it is described here as a new species, E. heptunei Denys et Van Bonn. The new species occurs in more southern and warmer waters than E. oiketis and also appears to be the first endemic diatom reported from Atlantic bottlenose dolphins.