Acetylcholinesterase (AChE) activity, a marker of neural differentiation, was histo-chemically localized in embryos and larvae of Mytilus galloprovincialis Lamarck and M. edulis L. (Mollusca: Bivalvia). The results show that: (1) the first AChE-active cells develop as two bilaterally symmetrical, postero-dorsal pioneer sensory neurons and their supporting superficial cells; (2) the pathways of their pioneering longitudinal axons are the same as those of two bilaterally symmetrical nerve cords which differentiate later in a postero-anterior direction; (3) in the conchostoma larva the posterior neurons and their associated superficial AChE-active cells give rise to two ciliary sensory-like organs which are the earliest rudiments of pedal ganglia and primary byssus glands; (4) three pairs of posttrochal gangliar rudiments, most probably visceral, parietal and pleural ganglia, develop on the same longitudinal nerve cords in the trochophore larva; (5) starting from the veliger stage, the presumptive foot rotates on the ventral side and migrates forwards, so that this gangliar chain is bent and the viscero-parietal ganglia become localized dorso-posteriorly to the pedal ganglia; (6) the cerebral ganglia differentiate together with the apical organ and the nerve network of the velum after the first pedal nerve rudiments have been detected.