Abstract: Chlamydomonas reinhardtii, a unicellular eukaryotic green alga, has been a powerful model system for understanding several fundamental biological processes such as photosynthesis and plant cell-cycle regulation. An artificial cell-cycle synchronization system is necessary for these studies. Here, we investigated a new method for synchronizing Chlamydomonas cell-wall-less cells, with which a high level of synchrony can be achieved. The degree of synchrony was analysed by flow cytometry. The cells are grown asynchronously in continuous light, then grown for 24 h in the dark (dark-shifting a continuous-light culture). At the end of the dark period, most cells (>80%) are arrested in the G1 phase. If necessary, an S-phase blocking agent can be used to treat the dark-shifted cells. When the dark-shifted cells are treated with M-phase blocking agent alone or with the S-phase and M-phase blocking agent sequentially, a cell population with more than 80% of cells at the M phase can be achieved. A specific synchronous procedure needs to be selected according to the properties of a certain Chlamydomonas strain (especially certain mutants) and the requirement of research.