An account is given of some viruses affecting insects of economic importance. Twenty-six viruses are listed affecting twenty-two species of insects. Of these viruses eleven cause cytoplasmic, and eight cause nuclear, polyhedroses. Six are granulosis viruses and one virus has no intracellular inclusions. Most of these viruses are new or recently recorded. The granulosis viruses, affecting Pieris brassicae, P. rapae and Melanchra persicariae, and the diseases they cause, are described in some detail.

We are indebted to Dr L. L. J. Ossowski of the Wattle Research Institute, University of Natal, and to Mr T. H. Coaker of the Cotton Research Station, Namulonge, Uganda, who sent us, respectively, diseased specimens of the wattle bagworm and the American cotton bollworm, from which the viruses were isolated. Miss S. Vernon-Smith took the electron micrographs, and Mr Simon Frey the photographs on the optical microscope.