Exposure of Saccharomyces cerevisiae to high osmotic stress evokes a number of adaptive changes that are necessary for its survival. These adaptive responses are mediated via multiple mitogen-activated protein kinase pathways, of which the high-osmolarity glycerol (HOG) pathway has been studied most extensively. Yeast strains that bear the hsp82T22I or hsp82G81S mutant alleles are osmosensitive. Interestingly, the osmosensitive phenotype is not due to inappropriate functioning of the HOG pathway, as Hog1p phosphorylation and downstream responses including glycerol accumulation are not affected. Rather, the hsp82 mutants display features that are characteristic for cell-wall mutants, i.e. resistance to Zymolyase and sensitivity to Calcofluor White. The osmosensitivity of the hsp82T22I or hsp82G81S strains is suppressed by over-expression of the Hsp90 co-chaperone Cdc37p but not by other co-chaperones. Hsp90 is shown to be required for proper adaptation to high osmolarity via a novel signal transduction pathway that operates parallel to the HOG pathway and requires Cdc37p.