In the eyes of insects as in the eyes and the skin of poikilitherm vertebrates, the presence of a tetrahydrobiopterin-derivative has been shown. In the blue-green alga, Anacytsis nidulans, a tetrahydropterin occurs which is highly active with Crithidia fasciculata but which is not identical to that of the animal source; its extraction requires special precautions. The yellow dihydropterin which is also similar, although not identical to that of the animal source, could be reduced by TPNH and the dihydrofolic-acid-reductase of chicken liver to the tetrahydro-level. On incubation with alkaline phosphatase the degradation products of the dihydro- and tetrahydropterin of A. nidulans, unlike that of the animal source, split off inorganic phosphate. The widespread occurrence of the dihydro- and tetrahydropterins, as well as that of Crithidia-active material not yet clearly identified, is discussed in connection with experiments which indicate their role as hydrogen carriers and as key-compounds in pterin metabolism.