The kaiABC gene cluster is important for maintaining circadian rhythms in the cyanobacterium Synechococcus PCC 7942. An extensive PCR-based survey of phylogenetically diverse cyanobacteria was conducted using degenerate primers designed to identify the presence of the kaiC gene. Hybridization and sequence analyses showed that the observed amplification products had a high degree of similarity to kaiC. Forty cyanobacterial strains possessed kaiC related sequences, suggesting that a clock system is universal among cyanobacteria.