Many cyanobacteria have a sophisticated circadian clock based on an elegant biochemical system requiring only three protein components: KaiA, KaiB, and KaiC (2). However, genome sequencing has revealed that the most abundant cyanobacterium of all, the marine cyanobacterium Prochlorococcus marinus, possesses KaiB and KaiC but lacks KaiA (9). In this issue, Axmann et al. show that the Prochlorococcus KaiC protein in vitro possesses an enzymatic activity suggesting that it could be a functioning component of a timing mechanism (1). Those authors also provide convincing evidence that Prochlorococcus once had KaiA but lost it relatively recently. They suggest that compensating mutations of KaiC allow the KaiBC system to retain some timing function in the absence of KaiA. Prochlorococcus appears not to have a robust circadian clock, but it may have a timing mechanism that runs for one diurnal cycle (4). Axmann et al. suggest that KaiB and KaiC form the basis for this simplified clock in Prochlorococcus (1). Their study raises fascinating questions about the function, origins, and evolution of a circadian clock in cyanobacteria and other phototrophs.