Previous studies indicated that the developing fish spinal cord was a simple system containing a small number of distinguishable neuronal cell types (Eisen et al., Nature 320:269-271, '86; Kuwada, Science, 233:740-746, '86). To verify this we have characterized the cellular anatomy of the spinal cord of developing zebrafish in order to determine the number, identities, and organization of the spinal neurons. Spinal neurons were labeled by intracellular dye injections, application of an axonal tracer dye to all or subsets of the axonal tracts, and application of antibodies which recognize embryonic neurons. We found that nine classes of neurons could be identified based on soma size and position, pattern of dendrites, axonal trajectory, and time of axonogenesis. These are two classes of axial motor neurons, which have been previously characterized (Myers, J. Comp. Neurol. 236:555-561, '85), one class of sensory neurons, and six classes of interneurons. One of the interneuron classes could be subclassified as primary and secondary based on criteria similar to those used to classify the axial motor neurons into primary and secondary classes. The early cord (18-20 hours) is an extremely simple system and contains approximately 18 lateral cell bodies per hemisegment, which presumably are post-mitotic cells. By this stage, five of the neuronal classes have begun axonogenesis including the primary motor neurons, sensory neurons, and three classes of interneurons. By concentrating on these early stages when the cord is at its simplest, pathfinding by growth cones of known identities can be described in detail. Then it should be possible to test many different mechanisms which may guide growth cones in the vertebrate central nervous system (CNS).