The mol. wt of the three double-stranded RNA (ds-RNA) components of Aspergillus foetidus virus S have been re-determined by using polyacrylamide gel electrophoresis (a) of native ds-RNA with ds-RNA standards and (b) of denatured ds-RNA with single-stranded RNA standards. Good agreement between the two methods and confirmation that the relationship between log mol. wt. and electrophoretic mobility of ds-RNA is not linear were obtained. The new mol. wt. values (4-0 to 4-2 x 10(6); 2-5 to 2-7 x 10(6) and 0-26 to 0-28 x 10(6), resepectively) are significantly higher than those determined previously by linear extrapolation from reovirus ds-RNA standards.