This volume contains seven chapters, based on papers presented at a Symposium on Insect Viruses, held in conjunction with the 67th Annual Meeting of the American Society for Microbiology in New York, N. Y. , on 30 April-4 May, 1967. The Symposium was organized to bring together outstanding workers interested in various areas of insect virology, and allow an opportunity for a discussion of problems, approaches, and methods that would lead to further progress in basic and applied research. One of the princi- pal reasons for holding the Symposium at this time was the feeling that the divergent areas of research, up to now studied separately by entomologists, medical and public health workers, geneticists, and plant pathologists, would be brought together, crossing the artificial borders and finding new, exciting and inspiring vistas. Insect viruses provide a rare opportunity to get acquainted with the work and methods of investigators in such related and yet distant fields. Following the symposium, a decision was made to publish the papers in a single volume, extending the contents to provide a complete and scholarly review of each subject. Since viruses affecting insects have received little attention until recent years, it was felt that a fully documented presentation of diverse areas of insect virology merited publication. The invited authors, all recognized authorities in their respective fields, prepared their contri- butions in such a way that each is a concise unit.