Abstract
Examples of circadian rhythms have been described in eukaryotic organisms and in photosynthetic bacteria, but direct proof of their existence in other prokaryotes is limited and has been largely ignored. The aim of this article is to review existing evidence and to present preliminary results that suggest that the heterotrophic bacteriumPseudomonas putidashows regular variations in its growth pattern synchronized with light/darkness cycles. We put forward the hypothesis that circadian regulation of certain processes can take place in non-photosynthetic prokaryotes and may represent an adaptative advantage in specific environments.