Cytoarchitectural analyses combined with injections of the tracer horseradish peroxidase in various structures in the brain of the goldfish, Carassius auratus, have defined some of the major components of acoustic and lateral line mechanosensory circuits between the medulla and midbrain. The main acoustic receptor in Carassius, the saccule, is known to provide a major input to the dorsomedial zone of the descending octaval nucleus. The dorsomedial zone in turn projects bilaterally to the secondary octaval population (SO) and to nucleus centralis of the torus semicircularis. The SO is composed of three major subdivisions which are also present in a related otophysan, the catfish Ictalurus punctatus. The SO in Carassius projects bilaterally to nucleus centralis and to the saccular recipient zones of the ipsilateral descending octaval nucleus. By contrast, the mechanosensory lateral line receptors are known to direct most of their input to nucleus medialis. Nucleus medialis in turn projects bilaterally to nucleus praeeminentialis, nucleus ventrolateralis of the torus semicircularis, and the optic tectum, and to the contralateral nucleus medialis. We also provide evidence for a bilateral projection of nucleus medialis to the sensory trigeminal nucleus, and for a reciprocal input from the sensory trigeminal nucleus to the ipsilateral nucleus medialis.