Flagellar beating in Chlamydomonas was found to be activated by mechanical stimulation. Immediately after a wild-type cell suspension was vortexed, the average swimming velocity of cells increased from 130 μm/second to 150 μm/second, due to an elevation of flagellar beat frequency from ∼60 Hz to ∼70 Hz without detectable change in the flagellar waveforms. This response required outer arm dynein. Treatment with EGTA, Ca2+-channel blockers, or mechanosensitive-channel blockers inhibited it. In demembranated and reactivated cell models, a modest increase in Ca2+ concentration elevated the axonemal beat frequency. These data indicate that the mechanical agitation increases beat frequency because it causes Ca2+ influx into flagella, which then activates outer arm dynein. Cell Motil. Cytoskeleton 2009. © 2009 Wiley-Liss, Inc.