Immotile cilia of crown cells at the node of mouse embryos are required for sensing of a leftward fluid flow1 that gives rise to the breaking of left-right (L-R) symmetry2. The flow-sensing mechanism has long remained elusive, however, with both mechanosensing and chemosensing models having been proposed1, 3–5. Here we show that immotile cilia at the mouse node respond to mechanical force. In the presence of a leftward flow, immotile cilia on the left side of the node bend toward the ventral side whereas those on the right side bend toward the dorsal side. Application of mechanical stimuli to immotile cilia along the dorsoventral axis by optical tweezers induced Ca2+ transients and degradation of Dand5 mRNA—the first known L-R asymmetric molecular events—in the targeted cells. The Pkd2 channel protein was found to be preferentially localized to the dorsal side of immotile cilia on both left and right sides of the node, and the observed induction of Ca2+ transients preferentially by mechanical stimuli directed toward the ventral side could explain the differential response of immotile cilia to the directional flow. Our results thus suggest that immotile cilia at the node sense the direction of fluid flow in a manner dependent on a flow-generated mechanical force.