The growth of several strains of bacteria was inhibited by D2O-media. The degree of inhibition was strain specific. The incorporation of 0.5% NaCl (w/v) to the D2O-medium decreased the inhibition of growth. No adaptation to better growth in deuterium was obtained in any of the strains tested; however a deuterium resistant mutant was obtained from one strain. Two strains tested formed enlarged, distorted cells when grown for more than eight generations in D2O-media. Cells grown in D2O-media and washed and resuspended in H2O-0.85% NaCl (w/v), were up to sixty times more sensitive to ultraviolet irradiation than control cells. Deuterium induces the occurrence of forward mutants in some strains. Many loci, but not all, showed an increase in the back-mutation rate over the spontaneous level, indicating some specificity of action by deuterium. Deuterated bases, obtained from a strain grown in D2O-medium, did not induce any phenotypic or genotypic effects when supplied to specific baserequiring strains. The amout of deuterium incorporated into the bacteria may be related to the mutagenic effect induced. The genotypic and phenotypic effects induced by deuterium are probably the result of an overall isotope effect.