Abstract DNA extracted from SV40 virus has been shown to comprise two components with different sedimentation coefficients. The fast, 21.2 S, component appeared to be a circular molecule and the slow, 16.1 S, component the linear molecule produced by opening the ring. The molecular weight of the DNA, as estimated from band width in equilibrium density gradient, was found to be 2.7 × 10 6, as compared to 3.2 × 10 6 as estimated from the sedimentation coefficient of the slow component. The general properties of the DNA were similar to those of polyoma DNA except for the base composition. SV40 DNA was found to contain 41% guanine plus cytosine, as compared to 48% for polyoma DNA.