Title: “Bacterial Thermotaxis by Speed Modulation” Abstract: One of the most important factors that affects bacterial migration and is sensitive to thermal changes is the bacterial swimming speed controlled by the rotation of the flagellar motors. In the natural habitats of bacteria, gradients often extend over relatively long distances such that their steepness is too small for bacteria to detect. We studied the bacterial behavior in such thermal gradients and found that they migrate along shallow thermal gradients due to a change in their swimming speed resulting from the effect of temperature on the intracellular pH. When nutrients are scarce the bacteria’s intracellular pH and consequently the swimming speed decreases with temperature, which causes them to drift towards the warm end of the gradient. However, when serine is added to the medium at concentrations >300M, the intracellular pH increases causing the swimming speed to increase continuously with temperature, and the bacteria to drift towards the cold end of the gradient. This directional migration is not a result of bacterial thermotaxis in the classical sense, because the steepness of the gradients is below the sensing threshold of bacteria. Nevertheless, our results show that the directional switch requires the presence of the bacterial sensing receptors which seem to be involved in regulating the intracellular pH. Additionally, it is also important to understand how thermal fluctuations and rate of thermal changes experienced by bacteria during their excursion in natural environments affect their run speed. To this end we have studied the dynamics of the bacterial flagellar motor’s speed in response to thermal fluctuations by tethering bacteria to a glass surface through their flagella. Our results show that under heavy load the response of the motor to fast linear thermal changes is instantaneous. However, when subjected to thermal fluctuations with varying frequency, they exhibit a resonant response to specific frequencies reflecting the complex internal dynamics of the motor.