Sodium azide slows the growth of Saccharomyces cerevisiae and induces a lag which increases steeply with the concentration. Serial transfers result in the emergence of a more resistant adapted strain. When the unadapted strain is plated on solid media containing azide, larger 'primary' colonies, and smaller, slower-growing, 'secondary' colonies appear. The secondary colonies come from a mutant strain which resembles the 'small colony' strain of Ephrussi in using what appears to be an anaerobic metabolic pathway even when supplied with oxygen. The properties of this strain are investigated. During the early stages of the lag which follows the inoculation of the parent strain into a liquid medium containing azide, there is an initial rapid production of the mutants, the number of which then remains substantially constant. The mutants are not responsible for the ultimate growth of the culture as a whole, which depends upon the eventual multiplication of the non-mutant cells giving primary colonies. The mutants are not resistant to the azide, and appear to be formed as a result of damage inflicted on those cells of the inoculum which are still capable of undergoing early division when inoculated into the azide medium.