The diel cycle is a key regulator of the cell cycle in many dinoflagellates, and may play a rate limiting role in bloom formation. However, the mechanisms by which the diel cycle entrains the cell cycle remain poorly understood. In this study we describe diel phasing of the cell cycle in the Florida red tide dinoflagellate, Gymnodinium breve Davis, determine the diel cue which serves to entrain the cell cycle, and provide evidence for the presence of cyclin dependent kinase, a cell cycle regulator which may be responsive to this cue. Four laboratory isolates from the west coast of Florida were compared. When grown on a 16:8 light:dark cycle, all isolates displayed phased cell division, with S-phase beginning 6-8 h into the light phase, and mitosis following 12-14 h later, as determined by flow cytometry. A naturally occurring bloom of G. breve, studied over one diel cycle, displayed diel cell cycle phasing similar to that in the laboratory cultures, with S-phase beginning during daylight and the peak of mitosis occurring approximately four hours after sunset. In the laboratory cultures, the dark/light ôdawnö transition was found to provide the diel cue which serves to entrain the G. breve cell cycle, whereas the light/dark ôduskö transition did not appear to be involved. Evidence for the presence of cyclin dependent kinase (CDK) in G. breve was obtained using two approaches: (1) identification of a 34 kDa protein immunoreactive to an antibody against a conserved amino acid sequence (a -PSTAIR) unique to the CDK protein family and (2) inhibition of the cell cycle by olomoucine, a selective CDK inhibitor. Current work on signaling events which regulate cell cycle progression will be discussed.