The insulation of axons in the vertebrate nervous system by myelin is essential for efficient axonal conduction. Myelination disruption and remyelination failure can cause human diseases, such as multiple sclerosis and hereditary myelin diseases. However, despite progress in understanding myelination regulation, many important questions remain unanswered. To investigate the mechanisms underlying myelination in vivo, we generated transgenic zebrafish expressing enhanced green fluorescent protein (EGFP) under the control of the mbp promoter. This transgenic fish displayed faithful EGFP expression in oligodendrocytes and Schwann cells in embryonic and adult zebrafish. Interestingly, although myelination progressed continuously in the postembryonic central nervous system, some of the spinal cord regions were filled with unmyelinated axons even in the adult spinal cord, suggesting functional differences between myelinated and unmyelinated axons. Our results suggest that this transgenic zebrafish could be a valuable animal model to study oligodendrocyte differentiation and myelination in vivo.