Nodaviruses have emerged as major pathogens of a wide range of larval and juvenile marine finfish in aquaculture worldwide. The causative agents are non-enveloped, icosahedral, RNA viruses with diameters in the range of 25-34 nm. They display considerable serological and molecular homology, although the present evidence suggests that there is more than one agent causing disease in a range of species. The diseases produced by these nodaviruses invariably involve the central nervous system and the retina where they usually produce vacuolation and cell necrosis. Virus particles are numerous within the cytoplasm of affected cells and extracellularly. As a result of the lesions, affected larvae/juveniles exhibit a range of neurological signs usually culminating in high mortality rates (not uncommonly 100%). One virus, that of the European sea bass, has recently been cultured in a fish cell line, but to date techniques such as the fluorescent antibody test, enzyme-linked immunosorbent assay and polymerase chain reaction have relied upon the harvest of purified viral antigen from infected tissues rather than obtaining these reagents from viruses grown in cell cultures. The epidemiology of these diseases is only partly understood. All appear to transmit readily by cohabitation of infected fish with naive larvae or juveniles, but vertical transmission has only been recognized with striped jack nervous necrosis and sea bass nervous necrosis viruses. Consequently, some aspects of disease control are based on first principles, rather than application of a full understanding of epidemiological factors.