Startle behaviors in teleost fishes are well suited for investigations of mechanisms of sensorimotor integration because the behavior is quantifiable and much of the underlying circuitry has been identified. The teleost C-start is triggered by an action potential in one of the two Mauthner (M) cells. To correlate C-start behavior with electrophysiology, extracellular recordings were obtained from the surface of the medulla oblongata in the hindbrain, close to the M-axons, in freely swimming goldfish monitored using high-speed video. The recordings included action potentials generated by the two M-axons, as well as neighboring axons in the dorsal medial longitudinal fasciculus. Axonal backfills indicated that the latter originate from identifiable reticulospinal somata in rhombomeres 2-8 and local interneurons. Diverse auditory and visual stimuli evoked behaviors with kinematics characteristic of the C-start, and the amplitude of the first component of the hindbrain field potential correlated with the C-start direction. The onset of the field potential preceded that of the simultaneously recorded trunk EMG and movement initiation by 1.08+/-0.04 and 8.13+/-0.17 ms, respectively. A subsequent longer latency field potential was predictive of a counterturn. These results indicate that characteristic features of the C-start can be extracted from the neural activity of the M-cell and a population of other reticulospinal neurons in free-swimming goldfish.