Abstract. Fluid shear stress has been demonstrated to affect the structure and function of various cell types. In mammalian cells, it was hypothesized that shear-induced membrane fluidization leads to activation of heterotrimetric G-proteins. The purpose of this study was to determine if a similar mechanism exists in the dinoflagellate Lingulodinium polyedrum, a single-celled eukaryotic aquatic organism that bioluminesces under shear stress. Membrane fluidity changes in L. polyedrum were monitored using the molecular rotor 9-(dicyanovinyl)-julolidine, whose fluorescence intensity changes inversely with membrane fluidity. Dual-staining with 9-(dicyanovinyl)-julolidine and the membrane dye 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene p-toluenesulfonate indicates membrane localization. Subjecting L. polyedrum cells to increasing shear stress reversibly decreased 9-(dicyanovinyl)-julolidine fluorescence, while autofluorescence of the cytoplasmic chlorophyll did not change. The relationship between shear stress (0.63Pa, 1.25Pa, 1.88Pa, and 2.5Pa) and membrane fluidity changes was linear and dose-dependent with a 12% increase in fluidity at 2.5Pa. To further explore this mechanism a membrane fluidizing agent, dimethyl sulfoxide was added. Dimethyl sulfoxide decreased 9-(dicyanovinyl)-julolidine emission by 4115% and elicited a dose-dependent bioluminescent response at concentrations of 0.2%, 0.5%, 1.0%, and 1.25%. This study demonstrates a link between fluid shear stress and membrane fluidity, and suggests that the membrane is an important flow mechanosensor of dinoflagellates.