Many bacterial species swim using. agella. The. flagellar motor couples ion. ow across the cytoplasmic membrane to rotation. Ion flow is driven by both a membrane potential (Vm) and a transmembrane concentration gradient. To investigate their relation to bacterial. agellar motor function we developed a. fluorescence technique to measure Vm in single cells, using the dye tetramethyl rhodamine methyl ester. We used a convolution model to determine the relationship between. fluorescence intensity in images of cells and intracellular dye concentration, and calculated Vm using the ratio of intracellular/ extracellular dye concentration. We found Vm = -140 6 14 mV in Escherichia coli at external pH 7.0 (pHex), decreasing to -85 6 10 mV at pHex 5.0. We also estimated the sodium-motive force (SMF) by combining single-cell measurements of Vm and intracellular sodium concentration. We were able to vary the SMF between -187 6 15 mV and -53 6 15 mV by varying pHex in the range 7.0 -5.0 and extracellular sodium concentration in the range 1-85 mM. Rotation rates for 0.35-mu m-and 1-mu m-diameter beads attached to Na+-driven chimeric. agellar motors varied linearly with Vm. For the larger beads, the two components of the SMF were equivalent, whereas for smaller beads at a given SMF, the speed increased with sodium gradient and external sodium concentration.