During locomotion animals continuously adjust their movements to navigate complex and cluttered environments. Such adjustments are made possible through sensory feedback. One key modality providing feedback on movement is mechanosensation. In fishes there are vast arrays of mechanosensors on the surface of the body. These sensors allow fish to sense contact and movement of body elements. Here we examine one type of these surface mechanosensors, the Rohon-Beard (RB) neurons, which are spinal neurons of fish and aquatic amphibians and are the predominant surface mechanosensors in larval stages of development. In larval zebrafish, RBs are distributed along the length of the spinal cord and have elaborate arborizations into the skin. Here, we describe the anatomy of RBs in zebrafish larvae and investigate their function. We show that RBs are rapidly adapting cells that can signal the onset and offset of mechanical stimulation and can use rate and temporal coding mechanisms to encode stimulus intensity. At rest, the intensity of the mechanical stimulation influences the reliability and the latency of the swim response. During ongoing swimming bouts, data suggest that the location, amplitude and timing of the mechanical stimulation lead to differences in the modulation of motor output. We find that a high intensity stimulus during swimming activity can interrupt and reset the rhythm of cyclic activity but a lower intensity stimulus at the same location has no effect on the periodicity of cyclic bursting. By understanding the mechanosensory mechanisms of modulation we can gain insight into how fish and other organisms generate the adaptive movements that allow them to perform well in structure-rich and changeable environments.