Podocystis adriatica (Kützing) Ralfs in Pritehard is reported from Senegal as a host specific epiphyte on the red alga Heterosiphonia crispella (C. Agardh) Wynne. The diatom was common on six collections of H. crispella from three sites, and absent on ca. 400 collections of about 100 species of subtidal Chlorophyta, Phaeophyta and Rhodophyta examined in a floristic survey. This is the first documented account of apparent host-specificity in an epiphytic diatom.