This is the first report that adaptive mutagenesis can arise by chromosomal nondisjunction, a phenomenon previously associated exclusively with DNA alterations. We previously uncovered a novel regulatory mechanism in Candida albicans in which the assimilation of an alternative sugar, l-sorbose, was determined by copy number of chromosome 5, such that monosomic strains utilized l-sorbose, whereas disomic strains did not. We present evidence that this formation of monosomy of chromosome 5, which is apparently a result of nondisjunction, appeared with increased frequencies after a selective condition was applied, i.e., by adaptive mutagenesis. The rate of formation of l-sorbose-utilizing mutants per viable cell per day ranged from 10(-6) at the initial time of detection to 10(-2) after 4 days of incubation on the selective plate.