Directed locomotion requires coordinated motor activity throughout an animal’s body. The nematode C. elegans, with only 302 neurons, offers an opportunity to understand how locomotion is organized by an entire motor system. We discovered that the mechanism that organizes undulatory locomotion in C. elegans is a novel form of sensory feedback within the motor circuit. Stretch-sensory feedback simply compels each body segment to bend in the same direction and shortly after the bending of the adjacent anterior segment. Remarkably, the entire sensorimotor loop operates is contained within a single (particularly sophisticated) type of neuron. We used microfluidics, optogenetics, calcium imaging, and modeling to show how stretch-sensory feedback is integrated into the motor circuit and how it explains the propagation of undulatory waves from head to tail. Our results point to a new framework for the organization of swimming and crawling gaits in worm undulatory locomotion.