The particle weight (molecular weight) of øNS11 was determined from the sedimentation coefficient, diffusion coefficient, and partial specific volume of the phage. The sedimentation coefficient of the phage (s020, w) is 416±2.7S. The diffusion coefficient (D20.w0), which was determined by quasielastic light scattering measurement, is (0.57 ±0.03) × 107 cm2/S The partial specific volume was determined by the mechanical oscillation technique to be 0.747± 0.007 cm2/S Based on these values, the particle weight of the phage was calculated to be (70.3±4.3)x10 daltons, which agrees well with the particle weight (69–72x10 daltons) estimated from the molecular weight of phage DNA and the content of DNA. The Stokes radius of the phage particle was calculated to be 37.7±2 nm and hydration of the phage was estimated to be 1.18 cm of dry phage. From the particle weight and the chemical composition of the phage, we estimated that one phage particle contains one double- stranded DNA molecule, 16,000 residues of fatty acid, 72 protein I molecules, 920 protein H, 42 protein HI, 48 protein IV, 290 protein V molecules, and 3,700 molecules of polyamines.