The early development of the cranial nerves of the sea lamprey, Petromyzon marinus L., was studied in embryos and early prolarvae by immunocytochemical techniques with the marker for post-mitotic neurons acetylatedtubulin. The trigeminal and facial nerves were first observed in embryos 9 days post fertilisation. The glossopharyngeal and vagal nerves appeared later, which indicates a rostrocaudal gradient in differentiation of branchiomeric nerves. The anterior and posterior lateral line, octaval and hypoglossal nerves also appeared in early developmental stages, but the ocular motor nerves were not observed in prolarvae. The present results indicate that, in comparison with cranial nerves and ganglia organisation reported in larval and adult lampreys, organisational changes occur in the cranial nerves between the prolarval and larval stages. One important change is the disappearance of the pharyngeal branch of the facial nerve, which was not previously reported to be present in larval and adult lampreys, whereas it had been observed in earlier developmental stages. Comparison of the present results with those from studies carried out in other vertebrate species, including the Japanese lamprey, suggests that the developmental pattern of the cranial nerves is conserved in agnathans and differs from that reported in other vertebrate groups. As maturation of lamprey eyes and extraocular muscles is completed at metamorphosis, there appears to be a correlation between the late development of eye-related cranial nerves in lampreys and the anatomical structures that they innervate.