The primary central projections of the anterior and posterior lateral line nerves were determined in the larval sea lamprey, Petromyzon marinus L. using silver staining methods and axonal transport of horseradish peroxidase (HRP). Lateral line fibers course in the dorsal portion of the octavolateral area. At the level of their entry in the medulla, all the lateral line fibers originate ascending and descending branches. The anterior lateral line nerve consists of three roots: dorsal, intermediate and ventral. Afferents of the dorsal root project to the ipsilateral dorsal nucleus. The intermediate and ventral roots project ipsilaterally to the lateral neuropil of the medial nucleus and to the cerebellar lamina, but they do not cross the midline. Contralateral projections of this nerve were not found in larvae. Primary afferents of the posterior lateral line nerve project ipsi- and contralaterally to the medial neuropil of the medial nucleus. Ascending fibers course in the ipsilateral medial nucleus, enter the cerebellar lamina, cross the midline and descend to the lateral neuropil of the contralateral medial nucleus. In the cerebellar lamina, the fibers of the anterior lateral nerve are located ventrolaterally to those of the posterior lateral nerve. Lateral line efferents were not observed in larvae.