Although the ascidian tadpole larva harbors a prospectively valuable prototype of the chordate nervous system, with extensively characterized neural plate cell lineages, the simple cellular composition of the resultant central nervous system (CNS) is not documented in detail. The average total number of cells in the larval CNS of Ciona intestinalis is 335 (range +/- 4, n = 3), 65 or 66 of which reside in the nerve cord of the tail. The estimates were made by tracing and counting the number of nuclei in serial semithin (1 micron) sections cut longitudinally through three larvae, fixed no later than 2 hours after hatching. Within a single fourth larva, L4, 266 cells constituted the CNS in the trunk region of the larva, 45 of which occurred within the visceral ganglion, 215 in the sensory vesicle, and 6 in the neck between the two. Each cell was assigned to one of thirteen categories. Most (182, roughly 68%) are classified as ependymal, a specialized non-neural cell peculiar to embryonic and larval chordates, from their position lining the cavities of the neural tube's elaborations or from clear similarities in the cytological appearance to those that do. Five cells are accessory cells of the sensory structures: three lens cells and a pigment-cup cell in the ocellus, and a single pigment cell in the otolith. Of the remaining 79 cells, 36 are sensory, 17 receptors in the ocellus and 19 presumed hydrostatic pressure receptors; these lie on the right and left sides of the sensory vesicle, respectively. Eighteen of the visceral ganglion cells have been tentatively classified as neurons, as have the remaining 25 cells which form two clusters in the posterior region of the sensory vesicle.