Aspergillus foetidus virus S (AfV-S) and A. foetidus virus F(AfV-F) have been shown to be serologically unrelated. Amino acid compositions of the two virus capsids are compared and their capsid polypeptides have been examined by SDS-polyacrylamide gel electrophoresis. AfV-F contained one major (phi3) and two minor (phi1 and phi2) polypeptides with mol. wt. 87000, 125000 and 100000, while AfV-S contained one major (sigma1) and one minor (sigma2) polypeptide with mol. wt. 83000 and 78000 respectively. Evidence is presented that sigma2 may be derived from sigma1 polypeptide by proteolytic degradation in vitro. The mol. wt. of AfV-F4 and AfV-S1a particles were found from sedimentation and diffusion coefficients to be 13 I times 10-6 and 12 4 times 10-6 respectively. AfV-F capsid was estimated to contain 120 molecules of polypeptide phi3 and one molecule each of polypeptides phi1 and phi2, while AfV-S capsid was estimated to contain 120 molecules of polypeptide phi1. It has been shown that SIa and S-2a particles each contain a molecule of double-stranded RNA with mol. wt. 2 24 times 10-6 (RNA-224) and 2 76 times 10-6 (RNA-276) respectively, whereas S-Ib and S-2b particles each contain a molecule of RNA-224 and RNA-276 respectively, together with an additional molecule of double-stranded RNA of mol. wt. 0 I times 10-6. Evidence is presented that S4 particles contain two molecules of RNA-224. S3 particles gave only RNA-224 on extraction, but contain the equivalent of 11/2 molecules of RNA-224; the nature of these particles and other possible virus replicative intermediates is discussed. Double-stranded RNA of mol. wt. I 24 times 10-6 was derived from a newly described particle class, Fo.