This study demonstrates the presence of a relatively extensive but previously unrecognized nervous system in embryonic stages of the opisthobranch mollusc Aplysia californica. During the trochophore stage, two pairs of cells were observed to be reactive to antibodies raised against the neuropeptides FMRFamide and EFLRIamide. These cells were located in the posterior region of the embryo, and their anterior projections terminated under the apical tuft. As the embryos developed into veliger stages, serotonin-like immunoreactive (LIR) cells appeared in the apical organ and were later observed to innervate the velum. Also, aldehyde-induced fluorescence indicative of catecholamines was present in cells in the foot, oral, and possibly apical regions during late embryonic veliger stages. Just before the embryo hatches as a free-swimming veliger, additional FMRFamide-LIR and catecholamine-containing cells appeared in regions that correspond to the ganglia of what will become the adult central nervous system (CNS). Neurons and connectives that will contribute to the adult CNS appear to develop along the pathways that are pioneered by the earliest posterior FMRFamide-LIR cells. These observations are consistent with the hypothesis that, besides their presumed roles in the control of embryonic behaviors, some elements may also guide the development of the CNS. Embryonic nervous systems that develop prior to and outside of the adult CNS have also been reported in pulmonate and prosobranch species of molluscs. Therefore, the demonstration of early developing neurons and their transmitter phenotypes in A. californica presents new opportunities for a better understanding of the ontogeny and phylogeny of both behavioral and neuronal function in this important model species.