Small heat shock proteins (sHsps) show a very rapid stress- and mitogen-dependent phosphorylation by MAPKAP kinase 2. Based on this observation, phosphorylation of sHsps was thought to play a key role in mediating thermoresistance immediately after heat shock, before the increased synthesis of heat shock proteins becomes relevant. We have analysed the phosphorylation dependence of the chaperone and thermoresistance-mediating properties of the small heat shock protein Hsp25. Surprisingly, overexpression of Hsp25 mutants, which are not phosphorylated in the transfected cells, confers the same thermoresistant phenotype as overexpression of wild type Hsp25, which is either mono- or bis-phosphorylated at serine residues 15 and 86 within the cells. Furthermore, in vitro phosphorylated Hsp25 shows the same oligomerization properties and the same chaperone activity as the nonphosphorylated protein. No differences between phosphorylated and nonphosphorylated Hsp25 are detected in preventing thermal aggregation of unfolding proteins and assisting refolding of denatured proteins. The results suggest that chaperone properties of the small heat shock proteins contribute to the increased cellular thermoresistance in a phosphorylation-independent manner.