A diverse set of circadian clock mutants was isolated in a cyanobacterial strain that carries a bacterial luciferase reporter gene attached to a clock-controlled promoter. Among 150,000 clones of chemically mutagenized bioluminescent cells, 12 mutants were isolated that exhibit a broad spectrum of periods (between 16 and 60 hours), and 5 mutants were found that show a variety of unusual patterns, including arrhythmia. These mutations appear to be clock-specific. Moreover, it was demonstrated that in this cyanobacterium it is possible to clone mutant genes by complementation, which provides a means to genetically dissect the circadian mechanism.