Summary Many naturally occurring mutagens have been discovered either from a sudden outbreak of toxicosis in livestock and poultry or from the frequent incidence of hepatomas or cancers among inhabitants of certain districts. These are pyrrolizidine and some other alkaloids contained in higher plants (Senecio, Crotararia, Heliotropium, etc.), cycasin in cycads, bracken toxins in ferns and several kinds of mycotoxins produced by fungi. Chemical structures and major biological effects of those substances are discussed in relation to their carcinogenicity and mutagenicity. Some, for instance cycasin and sinigrin, are not mutagenic by themselves but their enzymatic metabolites are known to be mutagenic. From the view-point of genetic hygiene the most important group may be that of mycotoxins, because those are contained as contaminants of several sources in human food-stuffs. Since species and strains of fungi that produce toxic substances are so large in number and their chemical structures vary widely, it is almost impossible to describe the general features of those substances. Mutagenic investigations of those substances are as yet very limited, but increasing evidence for a close relationship between carcinogenicity and mutagenicity suggests that most of them are mutagenic. In view of the probable significance of mycotoxins on the incidence of spontaneous mutations as well as etiology of cancel, the necessity for further investigation is stressed.