Circadian rhythms are known to exist in all groups of eukaryotic organisms as well as oxygenic photosynthetic bacteria, cyanobacteria. However, little information is available regarding the existence of rhythmic behaviors in prokaryotes other than cyanobacteria. Here we report biological rhythms of gene expression in a purple bacterium Rhodobacter sphaeroides by using a luciferase reporter gene system. Self-bioluminescent strains of Rb. sphaeroides were constructed, which produced a bacterial luciferase and its substrate, a long chain fatty aldehyde, to sustain the luminescence reaction. After being subjected to a temperature or light entrainment regime, the reporter strains with the luciferase genes driven by an upstream endogenous promoter expressed self-sustained rhythmicity in the constant free-running period. The rhythms were controlled by oxygen and exhibited a circadian period of 20.5 h under aerobic conditions and an ultradian period of 10.6-12.7 h under anaerobic conditions. The data suggest a novel endogenous oscillation mechanism in purple photosynthetic bacteria. Elucidation of the clock-like behavior in purple bacteria has implications in understanding the origin and evolution of circadian rhythms.