The nervous system of the postmetamorphic frog Xenopus laevis, like that of other amphibians, shows continued growth and a high capacity for regeneration, especially in its visual system. This characteristic has been attributed, in part, to the retention in adults of traits that in mammals are limited to embryos. In mammals, the progressive maturation of neurons is marked by successive changes in neuronal intermediate filament (nIF) subunit composition. For example, in mammalian forebrain, newly differentiating neurons first express the low molecular weight nIF protein alpha-internexin. As neurons mature, alpha-internexin expression declines, and expression of the low molecular weight neurofilament triplet protein (NF-L) increases. Thus, a systematic examination of the expression of low molecular weight nIF proteins in the postmetamorphic frog might reveal whether the nIF subunit composition of its neurons more closely resembles that of embryonic as opposed to adult mammals. Previously, X. laevis has been shown to express both NF-L and XNIF, a novel low molecular weight nIF protein that most closely resembles mammalian alpha-internexin. We have now discovered a new, low molecular weight nIF protein with even higher homology to alpha-internexin. We named this protein xefiltin, because it shared highest sequence identity with gefiltin, an alpha-internexin-like nIF protein from the goldfish visual system. In situ hybridization with probes to xefiltin, XNIF and NF-L showed that transcripts of all three were expressed widely throughout the post-metamorphic frog nervous system, but with distinctly different patterns of expression. For example, xefiltin was the most abundantly expressed of the three in retinal ganglion cells and in neurons of the habenular nucleus and telencephalon, whereas XNIF and NF-L were found at higher levels than xefiltin in peripheral sensory ganglia and in structures caudal to the mesencephalon. In general, the combined distributions of xefiltin and XNIF paralleled the distribution of alpha-internexin in mammalian embryos. Thus, we speculate that the persistence of alpha-internexin-like nIF proteins in the amphibian nervous system may be important for its continued potential for growth and plasticity.