The influence of phosphorus (P) starvation on the cell cycle of Prochlorococcus was studied for several strains from the Mediterranean Sea, the Atlantic and Pacific Oceans. P starvation could only be induced at high N/P ratio (>=150). In P-starved stationary phase, the fraction of cells in G1 dropped markedly, while cells accumulated not only in G2, but also in the DNA synthesis phase S, indicating that cells were not arrested at a precise cell cycle location but in all cell cycle phases. The percentage of P-starved cells in each phase varied slightly between strains. For the Mediterranean Sea strain CCMP 1378, cell cycle arrest in G1 and G2 could be reversed by addition of phosphates, while cells arrested in S could not resume their cycle and died. Such irreversible arrest in S had never been reported previously in phytoplankton. This suggests that cell cycle controls are not very tight in Prochlorococcus, since cells are not prevented from entering the critical S phase in the absence of phosphorus.