Some microalgae are sensitive to light intensity gradients. This property is known as phototaxis: The algae swim toward a light source (positive phototaxis). We use this property to control the motion of microalgae within a Poiseuille flow using light. The combination of flow vorticity and phototaxis results in a concentration of algae around the center of the flow. Intermittent light exposure allows analysis of the dynamics of this phenomenon and its reversibility. With this phenomenon, we hope to pave the way toward new algae concentration techniques (a bottleneck challenge in biofuel algal production) and toward the improvement of pollutant biodetector technology.