Cell proliferation of dinoflagellates is negatively affected by mechanical agitation and red tides caused by members of the group have been correlated with periods of calm sea conditions. The mechanism involved in the mechanically transduced inhibition of cell proliferation is thought to involve the disruption of the cell division apparatus. In this study, we used highly synchronized cells and flow cytometry to study the effects of mechanical agitation on cell cycle progression. We observed that mechanical agitation induced transient cell cycle arrest at G(1) phase, in both the heterotrophic dinoflagellate Crypthecodinium cohniiand the photosynthetic dinoflagellate Heteroscapsa triquetra.