The authors report the successful infection oí rhesus monkeys after inoculation of chikungunya virus by the intravenous and intramuscular routes. Monkeys could also be infected through the bite of infected Aedes aegypti. The animals developed a viraemia 1-2 days after inoculation and the viraemia usually lasted for 4 days during which time virus titres in excess of 7 log mouse LD50 were recorded. No apparent sickness was seen in any of the monkeys, all of which developed haemagglu-tination-inhibiting and neutralizing antibodies.