The plaque-reduction neutralization test (PRNT) was used to compare 15 isolates of Ockelbo virus from Sweden, one isolate of Ockelbo virus from Russia, the Egyptian prototype Sindbis virus, and Sindbis-like viruses from Slovakia, South Africa, Cameroon, and Australia. Strains from northern Europe (Sweden and Russia) were indistinguishable by PRNT. We observed some antigenic variation between isolates of Sindbis virus from Europe, Africa, and Australia. An Australian strain (C-377) was shown to be distinct from prototype Sindbis virus, and the Acrocephalus strain from Slovakia was shown to be identical to prototype Sindbis virus. All other strains, including Ockelbo virus isolates, were shown to be subtypes of prototype Sindbis virus.