Trehalose is thought to be important for desiccation tolerance in a number of organisms, including Saccharomyces cerevisiae, but there is limited in vivo evidence to support this hypothesis. In wild-type yeast, the degree of desiccation tolerance has been shown previously to increase in cultures after diauxic shift and also in exponential-phase cultures after exposure to heat stress. Under both these conditions, increased survival of desiccation correlates with elevated intracellular trehalose concentrations. Our data confirm these findings, but we have tested the apparent importance of trehalose using mutant strains with a deleted trehalose-6-phosphate synthase gene (tps1Delta). Although tps1Delta strains do not produce trehalose, they are nevertheless capable of desiccation tolerance, and the degree of tolerance also increases after diauxic shift or heat stress, albeit slightly less than in the wild type. Conversely, when wild-type yeast is subjected to osmotic stress, mid-exponential-phase cultures produce high concentrations of intracellular trehalose but show little improvement in desiccation tolerance. These results show that there is no consistent relationship between intracellular trehalose levels and desiccation tolerance in S. cerevisiae. Trehalose seems to be neither necessary nor sufficient for, although in some strains might quantitatively improve, survival of desiccation, suggesting that other adaptations are more important.