An agent, serologically identical to a Wyeomyia virus obtained from mosquitoes, was isolated from a worker on the inter-American highway project in Darien Province in eastern Panama. He experienced a mild febrile illness with recovery. A significant rise in anttibody titer to this virus was demonstrated in his serum during convalescence. Neutralizing antibodies to this newly isolated strain were found in 10 of 59 blood samples from inhabitants of Darien Province. The virus is designated the Darien strain.