Brief Summary1. My attempts to find why non‐irradiated Cod Liver Oil coming from a fish living at a depth of 100 meters or more in the sea, where very few rays are acting‐, should have an antirachitic action comparable to that of ultra violet light, give reason to the following explanation.2. The diatoms containing 8 % fat substances included in a shell of siliceous acid, that transmits the U.V.‐rays of the sun, are eaten of even big fish.3. Phytosterol is stated in a considerable amount in the extracts of these diatoms, which are living in the brightest layers of the sea, where the U.V.‐rays of the sun predominate and are acting on the ergosterol (cholesterol combined with fat acid?). The action of irradiation on the lipoidal substances in the living cells was studed by me in an Ultramicroscope with photographic film. In 10 minutes the transformation is clompleted in dark cells, others are »activated» before. The fat substances shine then like silver; the diatoms multiply in that stage – or they die by too long irradiation, still shining after the death. (Vitamins for the deep‐sea fish!?)4. The pigment fucoxanthin, peculiar to brown algae (Fucus, Diatoms) is suggested to be a sort of Bar for the 3 unsaturated carbon linkages in the activated ergosterol. The energy of the lipoid must then be liberated in the moment of depigmentation. The depigmentation is done in 10 minutes in extracts by U.V.‐rays or ozonisation in the dark. This is the time for the production of the effulgence stage of the living algae as well as of the maximal antirachitic action of C.L.O. The danger of too long“ irradiation is pointed out as concerne the antirachitic action. Inactive ozonids are formed (Saturation of unsaturated C‐atoms).5. The presence of 3 unsaturated carbon linkages in antirachitic (and other biological) processes seems to have a deeper energetic value. The relation Ca: P in blood serum by the formation of new bone and in the principal inorganic constituent of the bone is the same, 2:1. If disturbed, this relation is, as stated by others, reestablished (without other alteration of the food) by activated ergosterol (Windaus, Euler).6. The diatoms in question, in spite of being kept in the dark during 5 months possess a considerable antirachitic potency in contrast to green plants. The ev. vitamin D formation in fish is not confirmed.7. The Cod fish must reasonably have a considerable amount of accumulated energy from the sun, because he lives in the dark (more than the little caplin).