The neuromuscular system in the trunk of larval and adult zebrafishes was studied by means of light and electronmicroscopical methods. Spinal motoneurons were identified with the horseradish peroxidase retrograde transport method. We correlated the differentiation and growth of the myotomal muscle with the number of motoneurons per spinal cord segment and the size of the motoneuron somata. The adult number of motoneurons is reached in an early larval stage, before the muscle fiber type differentiation in the myotomes is completed. The mean motoneuron size does not bear a clear correlation with the size of the myotomal muscle. In adult zebrafishes we could distinguish the motoneurons which innervate the superficial slow red and the deep fast white muscle fibers on the basis of soma size and position in the motor column. The motoneurons of the red muscle part are small; they are located in the ventrolateral part of the motor column. The motoneurons of the deep fast white fibers are large; they lie near the central canal.