Transmission electron microscopy (TEM) and histochemical approaches were used to investigate the topology and ultrastructure of the nervous system of the tornaria larva of an enteropneust, Balanoglossus proterogonius. Cholinesterase activity was detected in the epithelium of the pre- and postoral ciliary bands. Groups of catecholamine-containing cells (CA) were detected at the anterior tip of larva, in the ventral epidermis behind the mouth, and in the stomach wall near its junction with the intestine. Single CA neurons were detected in the telotroch epithelium. Axon tracts are described in ciliary band epithelia. At the base of the aboral plate, epithelial nerve cells form a ganglion-like cluster. Single neuron-like cells and single axons and axonal tracts were found in the epithelium of digestive tract. The data were compared with ones from the literature and with those obtained from other marine invertebrate larvae. The properties of the neural elements and their possible functions are discussed.