The vertebrate spinal cord contains distinct classes of cells that form at precise dorsal-ventral locations and express specific combinations of transcription factors. In amniotes, V2 cells develop in the ventral spinal cord, just dorsal to motoneurons. All V2 cells develop from the same progenitor domain and hence are initially molecularly identical. However, as they start to become post-mitotic and differentiate they subdivide into two intermingled molecularly-distinct subpopulations of cells, V2a and V2b cells. Here we show that the molecular identities of V2a and V2b cells are conserved between zebrafish and amniotes. In zebrafish, these two cell types both develop into interneurons with very similar morphologies, but while V2a cells become excitatory Circumferential Descending (CiD) interneurons, V2b cells become inhibitory Ventral Lateral Descending (VeLD) interneurons. In addition, we demonstrate that Notch signalling is required for V2 cells to develop into V2b cells. In the absence of Notch signalling, all V2b cells develop as V2a cells.