Viruses isolated during 1979 and 1980 from patients with polyarthritis in New Caledonia and Wallis and Futuna Islands have been found to be more closely related to Ross River virus than any other regional Alphavirus. On the basis of virulence in suckling mice the majority of these isolates were found to be more closely related to the NB5092 strain of Ross River virus than to the prototype T48 strain.