Although numerous investigations have been carried out concerning the occurrence of vitamin D (D2 and D3) and their provitamins in different foodstuffs, about the effects of vitamin D intake on the human body as well as the cellular effects of the physiologically active form of vitamin D, there are almost no studies on vitamin D in an ecological context. One source for vitamin D is fish. But fish cannot synthesize vitamin D, nor provitamin D. Both originate at the beginning of the food chain, in phytoplankton. It is likely that the conversion of provitamin D (D2 and D3) to vitamin D can take place only under the influence of ultraviolet-B radiation in algae. Therefore the vitamin/provitamin ratio can perhaps be used as a much needed internal "exposure meter" for UV-B radiation. As is well known, provitamin D3 (7-dehydrocholesterol) can be converted to vitamin D3 also in human skin under the influence of ultraviolet-B radiation. Little is known about which species or groups of planktonic algae are capable of vitamin D synthesis, since only natural mixtures of algae and a few defined species have been analyzed. We have observed that reindeer lichen contains vitamin D2 and D3. For plant scientists vitamin D is interesting also because it is synthesized by some (but not all) higher plants, and acts as a growth substance in plants. Provitamin D2 (ergosterol) is synthesized by many fungi, and this fact may explain some traits of plant-fungal symbiosis.