Some properties of Drosophila C virus (DCV), a non-occluded isometric virus, have been studied. The virus particles were 30 nm in diam., their sedimentation coefficient was 153S and their buoyant density was 1.34 g/ml in caesium chloride in the pH range 7 to 9. These particles contained about 31% ribonucleic acid (RNA) and 69% protein. The reaction of formaldehyde with DCV particles suggested that the RNA in situ is single-stranded. The infectivity of DCV was stable at pH 3. The virus capsid contained three major polypeptides with mol. wt. of 31000, 30000 and 28000, and two minor components of mol. wt. 37000 and 8500.