The EFDL strain of Bluegill virus, a fish virus, was grown in a Bluegill Fry cell line (BF-2) in medium containing [3H]uridine or [32P]orthophosphate. This virus was purified from the cellular material by precipitation with PEG 6000 followed by isopycnic centrifugation in a Percoll gradient. As calculated from reconstruction experiments, only 8.5 and 0.5%, respectively, of the host proteins and the host RNA were recovered with the viral band, whereas the recovery of viral infectivity was approximately 20%. Electron microscopy of the viral band showed mostly intact virus particles. The nucleic acid extracted from purified virus was found to have the following properties: (i) it migrated homogeneously during electrophoresis at the rate expected from a single-stranded nucleic acid of molecular weight 2.17 × 106, (ii) it sedimentated as a single molecular species when analyzed by velocity centrifugation in sucrose gradients,(iii) it banded in a neutral Cs2SO4gradient at a density of 1.69 g/cm3as expected for single-stranded RNA, (iv) its base composition is that of a single-stranded RNA molecule: 14.89% C, 12.77% A, 37.03% U, and 35.23% G. The nucleic acid of Bluegill virus thus appears to be a single-stranded RNA molecule of approximately 2.17 × 106molecular weight.