Spinal interneurons coordinate the activity of motoneurons to generate the spatiotemporal patterns of muscle contractions required for vertebrate locomotion. It is controversial to what degree the orderly, gradual recruitment of motoneurons is determined by biophysical differences among them rather than by specific connections from presynaptic interneurons to subsets of motoneurons. To answer this question, we mapped all connections from two types of interneurons onto all motoneurons in a larval zebrafish spinal cord hemisegment, using serial block-face electron microscopy (SBEM). We found specific synaptic connectivity from dorsal but not from ventral excitatory ipsilateral interneurons, with large motoneurons, active only when strong force is required, receiving specific inputs from dorsally located interneurons, active only during fast swims. By contrast, the connectivity between inhibitory commissural interneurons and motoneurons lacks any discernible pattern. The wiring pattern is consistent with a recruitment mechanism that depends to a considerable extent on specific connectivity. Display omitted •Serial block-face electron microscopy dataset from larval zebrafish spinal cord•All motoneurons (MNs) in one hemisegment reconstructed, showing three subtypes•Specific connections from “fast-swim” interneurons (displaced CiDs) to large MNs•No speed-dependent MN specificity for commissural interneurons (CoBLs) Svara et al. use a three-dimensional electron microscopic dataset of larval zebrafish spinal cord to show that one type of interneuron (CiD), which drives MNs rhythmically, is partially selective in which motoneuron subtypes it contacts. This can explain mechanisms of orderly recruitment of MNs.