Understanding the neuronal networks in the spinal cord is hampered by the huge diversity of neurons and their connections. Transcription factors are beginning to provide a key to resolving classes of spinal interneuron and revealing homologies between different vertebrates. In developing zebrafish and frog, a discrete population of spinal interneurons with very characteristic axonal projections, express engrailed (En-1; Higashijima et al., 2002). We have used whole-cell recordings from pairs of neurons together with neurobiotin filling to study the responses and connections of these ascending interneurons in Xenopus tadpoles. We show directly that these neurons produce glycinergic, negative-feedback inhibition that can limit firing in neurons of the swimming central pattern generator. They may therefore be functionally related to mammal Renshaw cells that are also thought to express En-1. In the frog tadpole, these ascending interneurons have a second role. They are the modulatory neurons producing glycinergic inhibitory gating of cutaneous sensory pathway interneurons during swimming (Li et al., 2002). These discoveries raise the possibility that some classes of interneuron with distinct functions later in development may differentiate from an earlier class where these functions are shared.