Saccharomyces cerevisiae isolates have been shown previously to exhibit a high degree of variation in their ability to proliferate and persist in CD-1 mice (K.V. Clemons, J.H. McCusker, R. W. Davis, and D.A. Stevens, J. Infect. Dis. 169:859-867, 1994). Isolate origin was not a firm predictor of virulence phenotype, since the virulence phenotypes of clinical and nonclinical isolates ranged from virulent to avirulent and from intermediate to avirulent, respectively. Therefore, it was important to determine if there was any association between putative virulence traits and virulence that might help explain the variation in virulence phenotypes. S. cerevisiae isolates spanning a range of virulence phenotypes in experimental infections were examined for putative virulence traits: the ability to grow at supraoptimal temperatures (42, 39, and 37 degrees C), gelatin liquefaction, casein utilization, and pseudohyphal formation. Gelatin liquefaction appeared to be unrelated to pseudohyphal formation on casein or to virulence. Significant differences in the ability to grow at 39 and 42 degrees C were observed when the virulent and intermediate classes were compared with the avirulent class. Less extreme but still significant differences in pseudohyphal formation were observed when the virulent and intermediate classes were compared with the avirulent class. Therefore, two virulence traits, similar to those identified in other pathogenic fungi, the ability to grow at elevated temperatures and pseudohyphal formation, have been identified in S. cerevisiae.