The second messenger cAMP is a key regulator of growth in many cells. Previous studies showed that cAMP could reverse the growth inhibition of indoleamines in the dinoflagellate Crypthecodinium cohnii Biecheler. In the present study, we measured the level of intracellular cAMP during the cell cycle of C. cohnii. cAMP peaked during the G1 phase and decreased to a minimum during S phase. Similarly, cAMP-dependent protein kinase activities peaked at both G1 and G2+M phases of the cell cycle, decreasing to a minimum at S phase. Addition of N6, O2′-dibutyryl (Bt2)-cAMP directly stimulated the growth of C. cohnii. Flow cytometric analysis of synchronized C. cohnii cells suggested that 1 mM cAMP shortened the cell cycle, probably at the exit from mitosis. The size of Bt2-cAMP treated cells at G1 was also larger than the control cells. The present study demonstrated a regulatory role of cAMP in the cell cycle progression in dinoflagellates.