We have constructed an Escherichia coli strain lacking the small heat shock proteins IbpA and IbpB and compared its growth and viability at high temperatures to those of isogenic cells containing null mutations in the clpA, clpB, orhtpG gene. All mutants exhibited growth defects at 46°C, but not at lower temperatures. However, the clpA,htpG, and ibp null mutations did not reduce cell viability at 50°C. When cultures were allowed to recover from transient exposure to 50°C, all mutations except Δibpled to suboptimal growth as the recovery temperature was raised. Deletion of the heat shock genes clpB and htpGresulted in growth defects at 42°C when combined with thednaK756 or groES30 alleles, while the Δibp mutation had a detrimental effect only on the growth of dnaK756 mutants. Neither the overexpression of these heat shock proteins nor that of ClpA could restore the growth ofdnaK756 or groES30 cells at high temperatures. Whereas increased levels of host protein aggregation were observed indnaK756 and groES30 mutants at 46°C compared to wild-type cells, none of the null mutations had a similar effect. These results show that the highly conserved E. coli small heat shock proteins are dispensable and that their deletion results in only modest effects on growth and viability at high temperatures. Our data also suggest that ClpB, HtpG, and IbpA and -B cooperate with the major E. coli chaperone systems in vivo.