Competition assays confirm the greater fitness of mutant strains at 42°C in E. coli. The ancestor and one mutant strain were grown together in a two-stage chemostat on E. coli C at 37°C or 42°C. Concentration of each strain was determined by allele specific PCR at the beginning and end of each chemostat. The y-axis shows the average increase in frequency of the mutant strain after two hours. Three replicate chemostats were run for each mutant at each temperature. Mut323 is a non-synonymous substitution in C, and mut 324 is synonymous. The increase in mutant strain frequency was significantly different between the two temperatures for both mutants (Wilcoxon rank sum test, p < 0.005).