The properties of polyhedral double-stranded (ds)RNA virus particles obtained from 8 isolates of the wheat take-all fungus, G. g. var. tritici, were investigated. Thirteen viruses and virus variants were distinguished and classified into 3 groups on the basis of serological and physical properties of the virus particles; viruses in a group were related serologically to other members of the same group but were unrelated serologically to members of other groups. Group I viruses had particle diameters of 35 nm sedimenting at 109-126S; the virus capsid contained 1 polypeptide species, MW 54-60 .times. 103, and virus dsRNA consisted of 2-4 components, MW 1.0-1.3 .times. 106. Group II viruses had particle diameters of 3.5 nm sedimenting at 133-140S; the virus capsid contained 1 polypeptide species, MW 68-73 .times. 103, and virus dsRNA consisted of 2-4 components with MW 1.39-1.60 .times. 106. Group III viruses had particle diameters of 40 nm sedimenting at 159-163S; the virus capsid contained 3 polypeptide species, MW 78 .times. 103, 83 .times. 103 and 87 .times. 103, and virus dsRNA consisted of 2 components, MW 3.2-4.3 .times. 106. Five fungal isolates contained only single viruses. One isolate contained viruses from groups I and II, and 2 isolates each contained viruses from groups I, II and III. Investigation of virus and 10 single conidial isolates derived from 1 of the latter isolates showed that all 3 viruses were present in a single cell.