Crystal violet (hexamethyl para-rosaniline hydrochloride) in a solid medium delays the growth of a strain of Saccharomyces cerevisiae without very serious reduction in the viability of the cells. Growth in presence of the dye, in liquid or solid media, is influenced by marked co-operative effects (shown by the influence of inoculum size). Culture in a liquid medium containing crystal violet leads to the formation of mutant cells, slow-growing, small colony-forming and with respiratory deficiencies (shown by wasteful glucose utilization and failure to reduce tetrazolium chloride indicator). Both mutant and non-mutant cells are capable of adaptation to resist the action of the crystal violet, but there is no connexion between the development of the resistance and the occurrence of the mutation. Non-mutant cells which have been adapted to resist crystal violet are still capable of yielding mutants on further exposure to the action of the dye.