Muller's classical discovery in 1927 of the mutagenic action of X-rays has provided an extremely useful tool for studying the nature of gene mutation and chromosome breakage. It has always been realized that if, in addition, chemical substances were found capable of producing mutations and/or chromosome breaks a further important step forward in the analysis of the mutation process would follow. The search for such mutagenic substances has therefore been going on in various laboratories for some ten years, until recently, however, without definitely positive results. Only during the last few years has it been shown that certain chemical substances, such as mustard gas (ββ′-dichlordiethylsulphide, (ClCH2 · CH2)2S), may be as effective as X-rays in producing mutations and chromosome breaks (Auerbach, 1943; Auerbach and Robson, 1944; Auerbach, 1945; Auerbach and Robson, 1946). Indeed, the similarities between the results of these two types of treatment are impressive. In the course of more than four years during which work of this kind has been carried out by the author, only few differences between the genetical results of chemical treatment and of irradiation have come to light. Particular interest attaches to these dissimilarities, because they, rather than the many similarities, may throw some new light on the process of mutation and through this on the nature of the gene. One of the few striking dissimilarities is the high frequency of certain types of mosaics which can be produced by chemical treatment. It is the object of the present paper to summarize the data on chemically induced mosaicism, contrast them with similar data from X-ray experiments, and discuss the similarities and differences from the point of view of the mechanism of induced mutation.