The zebrafish has become an important model organism to study myelination during development and after a lesion of the adult central nervous system (CNS). Here, we identify Claudin k as a myelin-associated protein in zebrafish and determine its localization during development and adult optic nerve regeneration. We find Claudin k in subcellular compartments consistent with location in autotypic tight junctions of oligodendrocytes and myelinating Schwann cells. Expression starts in the hindbrain at 2 days (mRNA) and 3 days (protein) postfertilization and is maintained in adults. A newly generated claudin k:green fluorescent protein (GFP) reporter line allowed us to characterize oligodendrocytes in the adult retina that express Claudin k and olig2, but not P0 and uniquely only form loose wraps of membrane around axons. After a crush of the adult optic nerve, Claudin k protein levels were first reduced and then recovered within 4 weeks postlesion, concomitant with optic nerve myelin de- and regeneration. During optic nerve regeneration, oligodendrocytes, many of which were newly generated, repopulated the lesion site and exhibited increasing morphological complexity over time. Thus, Claudin k is a novel myelin-associated protein expressed by oligodendrocytes and Schwann cells from early stages of wrapping and myelin formation in zebrafish development and adult regeneration, suggesting important functions of the gene for myelin formation and maintenance. Our Claudin k antibodies and claudin k:GFP reporter line represent excellent ways to visualize oligodendrocyte and Schwann cell differentiation in vivo.