During the first 10 days of February 1975, an Australian hitchhiker contracted Marburg virus disease while traveling through Rhodesia and died; the infection was subsequently passed to two other persons, who recovered. Investigators retraced the hitchhiker's steps in March and again in June 1975 in an effort to uncover the natural reservoir of the virus and determine how it was transmitted. Serum samples were collected from humans and animals wherever the patient had come in close contact with animals or insects. Arthropods of various types were collected in June 1975 and again in February 1976 for virus isolation attempts; at no time did the patient come in direct contact with nonhuman primates of any kind, or any other animals. Indirect contact with bats, monkeys, and birds through aerosols was possible, though at some distance. Direct contact with arthropods occurred throughout the trip; on several occasions it was notably severe. We believe that during this outbreak the first Marburg virus infection occurred by vector-borne transmission from an arthropod yet to be identified, and that patients 2 and 3 acquired the disease by exposure to the oropharyngeal secretions of patients 1 and 2, respectively. Studies are underway to identify the species of arthropod involved in this transmission.