The precise ways in which the brain communicates with the spinal cord to generate selective motor output is little understood. We focus on a small group of spinal projection neurons (SPNs), which are known to play a necessary role in controlling turning behaviors in zebrafish. Here, we show that these neurons serve to bias the direction, amplitude and frequency of the first tail deflection of a swim bout, while the remainder of the swim remains unaffected and symmetrical. Furthermore, we find that the size of the turning angle correlates strongly with the graded activity of a subset of these neurons. Simulation studies support the notion that the target of these SPNs is a subset of commissural inhibitory spinal interneurons. Our findings show that the brainstem can control a switch in behavior, such as transforming forward swims into turns, by the graded activity of a small set of dedicated SPNs.