The nature of replicating instability was studied in an adenine-requiring strain of Schizosaccharomyces pombe. The instability was recognized by the presence of mosaic (part red, part white) colonies when cells of mosaic colonies, obtained by ethylmethanesulfonate (EMS) treatment, were replated. It was found that the instability was not more closely associated with the cells of the mutant white portion of primary mosaics than with those of the red portion. Data conformed to the supposition that replicating instability is expressed by mutations from unstable to stable “red” and to stable “white”. Calculations indicated that the spontaneous mutation rate from unstable to stable may be in the order of 0.2 to 0.4, per cell per generation.