The integument of marine mammals provides an ideal substrate for colonization by photosynthetic epizoans. A review of the literature and new data from the harbor seal (Phoca vitulina richardsi) revealed at least eight genera of photosynthetic cyanobacteria and algae colonizing the pelage of six species of pinnipeds. The relationship may be considered a facultative association, beneficial to the photosynthetic epizoans and relatively neutral (but perhaps slightly costly) to their mammalian hosts. We studied the incidence of algal growth on the pelage of harbor seals in central California during 1985–1987 and 2017. The green alga Ulva sp. occurred on adult females throughout the year and rarely occurred on adult males. Algal growth appeared to be most frequent during the breeding season (February to May) and least frequent during the molt and post-molt seasons (June to October). Algal growth occurred more frequently on harbor seals in estuaries than along the coast. Much remains to be learned about the colonization of pinnipeds by photosynthetic epizoans and why some seals are more susceptible to colonization by algae than others.