The reproductive fitness of cyanobacteria is altered by variation of temperature conditions and differential mutation of Kai genes that contribute to circadian rhythms. The degree and significance of these effects were tested in three distinct strains of Synechococcus elongatus cyanobacteria: a wild-type strain (AMC149) and two mutated strains with either an optimized or a disrupted circadian rhythm. These strains were allowed to compete in mixed cultures to determine the adaptive value of circadian clocks in relation to the reproductive fitness of cyanobacteria. Pure cultures served as controls, ensuring that growth rates were the same. Results illustrate that under constant light conditions at high temperatures, the strains with mutant circadian rhythms were both more reproductively fit than the wild-type strain. In conclusion, we found that circadian clocks do not confer a reproductive advantage for cyanobacteria when the cycle of the circadian clock does not match the light/dark cycle of the environment.