Growth of Rhodospirillum rubrum was followed in cultures kept under anoxic conditions at constant temperature in either continuous light (LL, 32°C) or continuous darkness (DD, 32°C and 16°C). In DD, only small modifications of the turbidity were detected; linear regression analysis nevertheless gives a very significant slope (t(34) = 13.07, p < 10-14, with R2 of 0.834). Mean generation times reflected these differences of growth with 11.9 ± 0.5 h in LL and 43.2 ± 1.1 h in DD at 32°C and 37.4 ± 1.0 hat 16°C cultures. The uptake hydrogenase (Hup) activity has been followed in situ in whole cells of R. rubrum grown in the same conditions, and a clear ultradian rhythm of activity has been observed. Indeed, after about 12 h in the new media, a rapid rise of hydrogenase activity was observed in both LL and DD cultures after which it decreased again to very low values. The activity of Hup continued to show such fluctuations during the rest of the experiment, both in DD and in LL, during the growth and stationary phases. The Lomb-Scargle power periodogram method demonstrates the presence of a clear rhythmic Hup activity both in LL and DD. In the LL-grown cultures, the oscillating activity is faster and continues throughout the growth and the stationary phases, with an ultradian period of 12.1 ± 0.5 h. In DD, the slow-growing bacteria showed an ultradian oscillatory pattern of Hup activity with periods of 15.2 ± 0.5 h at 32°C and 23.4 ± 2.0 h at 16°C. The different periods obtained for LL-and DD-grown bacteria are significantly different