The primary pathway descending to the spinal cord to initiate locomotion in the stingray is located in the intermediate to ventral portion of the lateral funiculus; a second pathway is located in the dorsolateral funiculus. The goal of this study was to identify the origins of these pathways in the rhombencephalic reticular formation (RF). To do this we used microstimulation of the RF in conjunction with selective lesions of the brain stem and spinal cord. In some animals microinjections of excitatory amino acids were used to avoid stimulating axons of passage. Locomotion in the contralateral pectoral fin was evoked by microstimulation of the dorsal and ventral reticular nuclei, the middle and superior RF, and the ventral portion of the lateral RF. The regions from which locomotion was evoked by chemical stimulation were more restricted and included the rostral dorsal reticular nucleus, the middle RF, and the adjacent ventral lateral RF. This area encompasses the magnocellular RF and coincides with the distribution of numerous reticulospinal cells that project ipsilaterally into the ventral half of the lateral funiculus. Our results indicate, then, that locomotion in the stingray is mediated primarily by a pathway originating in the magnocellular RF that descends ipsilaterally in the ventral half of the lateral funiculus to elicit swimming in the contralateral pectoral fin. We suggest that this primary pathway is specifically associated with the control of locomotion. We also demonstrated that locomotion can be evoked independently from the lateral RF, but is probably mediated by an indirect pathway relaying near the spinomedullary junction or in the rostral spinal cord.