Locomotion is implemented with a modular structure in the spinal cord. Upstream, the reticulospinal neurons (RSNs) receive various sensory inputs, descending control signals and ascending feedback from spinal circuits to command movement. Due to the limited surgical access and intermingling of excitatory and inhibitory cells in the region, it is unclear whether the brainstem RSNs shares a modular organisation similar to the spinal circuits. We address this question using larval zebrafish and focus on the central excitatory strip in the hindbrain labelled by transcription factor vsx2+ (V2a). Using Kaede-based optical backfills, we show the distributions of V2a-derived RSNs and quantify the number of projecting neurons to the rostral, middle and caudal level of the spinal cord. We revealed, with region-specific unilateral optogenetics, the functional structures of distinct V2a-derived locomotor modules : one referred to as forward module elicits symmetrical tail oscillations upon activation while the other referred to as the steering module elicits ipsilateral tail movement. To resolve the connectivity of the forward V2a-derived RSNs, we achieved single cell optogenetic mapping with holographic illumination combined with temporal focusing and simultaneous calcium imaging. Our results are consistent with recurrent connectivity among V2a-derived RSNs within the forward module which build up excitation in the RSN to trigger basic forward locomotion. Our findings provide a framework for comparative studies of the functional organisation of the RSNs across vertebrates, and highlight a key command region for forward locomotion in the medulla.