The numbers of 1) dorsal root ganglion cells in the 2nd spinal segment, 2) ventral horn cells in the 2nd spinal segment, 3) Purkinje cells of the cerebellum, and 4) neurons in the nucleus glomerulosus were counted and correlated with age and size in the guppy, Lebistes. The findings were that the neuronal numbers in all these areas increased throughout much of the life of the animal. These data, combined with the previously demonstrated increases in retinal neurons in goldfish and sensory and spinal neurons in stingrays, suggest that neurons are added to many areas of the nervous system of fish as the animal ages and grows. In this respect, the nervous systems of fish differ from the nervous systems of other vertebrates. We offer the suggestion that the comparatively greater ability of fish to regenerate their nervous system after injury may be related in part to their ability to add neurons to various parts of the nervous system throughout life.