Subsurface bioremediation is often hindered by the inability to achieve good mixing between injected bacteria and residual contaminants. Chemotaxis, which is the ability of bacteria to migrate preferentially toward higher concentrations of certain chemical attractants, could potentially increase bacterial transport into the contaminated zone. To observe and quantify this chemotactic enhancement to bacterial dispersion transverse to groundwater flow, a microfluidic device--a porous T-sensor-was created. It allowed two streams of equal flow rate to enter side-by-side into a porous channel; the transverse mixing of the two streams was controlled primarily by dispersion. When a suspension of the chemotactic bacteria Escherichia coli HCB1 and a solution of chemical attractant alpha-methylaspartate were injected as the two incoming streams, enhanced bacterial migration into the attractant stream was observed relative to a control experiment with dispersion alone. Chemotaxis was observed under lower flow rates comparable to natural groundwaterflow. The chemotactic response was greater than that predicted by an advection-dispersion equation model using a chemotactic coefficient derived under quiescent experimental conditions, which suggests that flow in porous media may further enhance transverse migration for chemotactic bacteria. This study provided direct evidence of the significance of bacterial chemotactic transverse migration at groundwater flow rates.