A number of physical properties of a cytoplasmic polyhedrosis virus of Heliothis armigera were determined. These included a buoyant density of 1.44 g/cm3, a diffusion coefficient of 0.73 × 10–7 cm2/sec, a partial specific volume of 0.688 ml/g, and a sedimentation coefficient of 399S. Three different methods were used to determine an average molecular weight of 47.4 ×106.