During studies on the virulence and genetics of isolates of Saccharomyces cerevisiae we noted colony phenotype switching. Virulent clinical (YJM128, YJM436) and avirulent non-clinical (Y55, YJM237) isolates of S. cerevisiae were scored for colony phenotypes and reversion rates of the switched second-generation colonies on YEPD with phloxine B. YJM128 produced six colony phenotypes; 99.7% were large (8-9 mm diameter), smooth, pink with a white edge (type A) and the other five types arose at frequencies ranging from 2 x 10(-3) to 5 x 10(-5). On replacing of type A, 98.8% of colonies scored arose as type A, 1.1% type J and 0.1% larger, but similar to type A indicating both stability and reversibility. YJM436 also varied, with 91.3% arising as a single phenotype; five other phenotypes were observed at frequencies ranging from 7.6 x 10(-2) to 1.5 x 10(-4). For Y55, 0.2% of the colonies were 5-7 mm in size with a ridged pink edge (type D); 36.4% of YJM237 were a 5-7 mm, smooth pink colony (type H). The remaining 99.8% of Y55 and 63.4% of YJM237 colonies arose as sectored or non-sectored non-reverting petites. These results indicate that virulent isolates of S. cerevisiae tend to produce multiple colony phenotypes, which are reversible. In contrast, avirulent isolates of S. cerevisiae showed single colony phenotypes and a high frequency of non-reverting petites. Whether selected colony phenotypes and switching are associated with the virulence of S. cerevisiae remains to be determined.