Mutualistic relationships of corals from warm, oligotrophic waters are best known with endocytic and scarcely with endolithic phototrophic organisms. We describe relationships of the microboring phototrophic green alga ostreobium queckettii and the cyanobacterium Plectonema terebrans with 2 species of strictly azooxanthellate corals from nutrient-rich cold waters. Desmophyllum dianthus was originally described from lightless bathyal habitats and has only recently been discovered to form vast and dense shallow-water populations in the Chilean fjord region where it is associated with Caryophyllia huinayensis. The filamentous phototrophic endoliths were found to live on the surface and bore into the uppermost layer of the corallites. They are markedly denser on portions of the corallites which are covered by polyp tissue. The infestation of corals appears in the form of yellowish, greenish or brownish staining in the case of O. queckettii, and pinkish to violet staining in the case of P. terebrans. These characteristics suggest a putative facultative and mutualistic ectosymbiosis. Similar relationships between phototrophic endoliths and D. dianthus are indicated in specimens from New Zealand.