The Caliciviridae are a family of small (35-40 nm) RNA viruses with a characteristic cupped morphology. They are unique in possessing only a single major structural polypeptide, of Mr 60,000-71,000. The use of electron microscopy to investigate diarrhoeal diseases has revealed viral particles with the size and structure of the caliciviruses in the faeces of humans, domestic and farm animals, birds, reptiles and insects. In vivo experiments indicate that they are species specific and have confirmed that they replicate in the gut, which often results in the host developing diarrhoea and failing to thrive. Biochemical characterization of these agents has been hampered by a failure to produce sufficient yields of virus in vitro. However, fluorescence and radiolabelling experiments indicate that the human, canine and chicken viruses replicate in the cytoplasm and possess an RNA genome. A major structural polypeptide (Mr 60,000-71,000) has been identified in the human, canine and insect viruses. Diagnosis of the candidate caliciviruses is dependent on electron microscopy and fluorescence labelling, with the exception of the human agents, for which radioimmunoassays have been developed. There is little epidemiological information on these agents but there is increasing evidence that the human caliciviruses are a common cause of outbreaks of diarrhoea and vomiting in infants, adults and the elderly.