Most organisms experience daily light-dark changes and show rhythms of basic biological processes such that they occur at optimal times of the day. By characterizing gene transcription and cell fluorescence rhythms in P. tricornutum, we recently uncovered the existence of circadian rhythms in diatoms and have demonstrated the involvement of the bHLH-PAS protein RITMO1 in the regulation of these processes. We are currently using RITMO1 as an entry point to identify other diatom circadian clock components and the input/output pathways. Physiological analysis of clock mutants will allow assessing the relevance of timekeeper mechanisms for diatom life.