Here we describe the cloning and expression pattern of Ci-isl, a homologue of vertebrate genes, in the ascidian. Early in development, Ci-isl expression occurs in the primordia of palps and brain vesicle, then in the tailbud embryo it is transiently extended to the notochord cells. At larva stage, the expression is down-regulated in the notochord, and it persists predominantly in the compartments of the nervous system. These observations indicate that also in invertebrates, islet genes show an expression pattern during differentiation of the nervous system.