Double-stranded nucleic acids from a strain of Penicillium chrysogenum containing RNA viruses were isolated by agarose-gel filtration, and separated into DNA and double-stranded RNA fractions by agarose-gel chromatography in 2.5m-NaCl. The DNA fraction contained less than 1% alkali-labile polynucleotides, and sedimented homogeneously at 8-10S in alkaline sucrose gradients. In CsCl gradients it tended to band in the density region of 1.66-1.72g/ml. It had a ;melting' temperature (T(m)) of 75 degrees C in 0.015m-NaCl-0.0015m-trisodium citrate, corresponding to 51.5mol% of G+C. The double-stranded RNA fraction did not contain detectable DNA. It could not band in CsCl up to a density of 1.78g/ml, and mainly consisted of a 14-15S RNA species with a T(m) of 88.5 degrees C in the above solvent, and a G+C content of 49.3 mol%.