The occurrence of Leptoseris fragilis at depths between 100 m and 145 m is extraordinary for a zooxanthellae-containing scleractinian coral species because of the low level of light available. Using different experimental techniques, it was possible to show structural and physiological adaptations which may be related to the occurrence of a photosynthetic coral in an extremely low light environment. A chromatophore system was detectable by TEM and SEM in the oral gastrodermis. The pigment granules (less than 1 /um in diameter) form a stratum on which the zooxanthellae lie in a monolayer arrangement; either the chromatophore system serves as a reflector and/or the pigment granules are involved in scattering. Thus, the probability of photons being absorbed by light harvesting pigments of the algae is increased. The oral epidermis contains large intercellular spaces which might facilitate light penetration. Pigment(s) present in the cytoplasm of the host seem(s) to be involved in a second mechanism which provides the algal symbionts with additional photosynthetically active photons. Light of short wavelengths outside the PAR-spectrum is absorbed by host pigment(s) and shifted into light of the PAR-spectrum, which is then emitted and subsequently utilized by the pigments of the zooxanthellae; this probably increases photosynthesis and the supply of photoassimilates to the host. In specimens transplanted for one year at a much higher light intensity to a depth of 40 m, the pigment granules were almost degenerated and the autofluorescence had nearly vanished.