SUMMARYThe RNA genome segments of thirty-three isolates of cytoplasmic polyhedrosis viruses (CPVs) were examined by polyacrylamide gel electrophoresis. Major differences were observed in the gel profiles of the RNA segments from many of the viruses; differences which were reinforced by polyacrylamide gel electrophoresis of the virus structural proteins. As a result of these studies, a provisional classification scheme for CPVs is proposed, where viruses with similar RNA gel profiles are included within the same 'type', while isolates differing in the molecular weights of most, or all of the RNA segments are assigned to different types. Using this system, eleven distinct CPV types were recognized. All eleven CPV types, like reoviruses, probably contain ten segments of RNA with a total mol. wt. of approx. I5 × Io 6.

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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.

SUMMARYA direct experimental comparison of the Tipula iridescent virus and the Sericesthis iridescent virus has shown them to be very similar but not identical. After extensive purification the viruses were differentiated by electrophoretic mobility, tryptic peptide patterns, serological cross-reactivity, and pathogenicity in a group of host insects. The viruses were identical in gross chemical composition and in a number of physical properties. A particle weight of i-z to I'3 x io 9 daltons was calculated by three methods. A slower sedimenting top component consisting of empty protein shells was found during the purification of each virus; similar empty shells could be produced by succinylation or formic acid-diphenylamine treatment of whole virus.

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