Abstract Uukuniemi virus, the prototype of a new serological group of arboviruses, was grown in BHK21 cells and purified by isopycnic centrifugation in sucrose gradients. The purified virus had a sedimentation constant of about 450 S. The buoyant density of the virus was 1.20 g/cm 3 in CsCl and 1.18 g/cm 3 in sucrose. Sucrose gradient centrifugation of SDS-treated virus gave two RNA peaks (21 and 27 S). Two polypeptides of molecular weight 65,000–75,000 and about 25,000 could be separated from whole virus by SDS-disc electrophoresis. Two fractions were obtained by CsCl gradient centrifugation of Nonidet-treated virus. A fraction with a density of less than 1.25 g/cm 3 contained phospholipids and the larger protein (molecular weight 65,000–75,000). This fraction apparently contains disrupted virus envelopes. The other fraction with a density of 1.31 g/cm 3 contained RNA and the smaller protein (molecular weight 25,000). Electron micrographs of the latter fraction showed strands about 2.5 nm thick and over 1000 nm long, apparently uncoiled nucleoprotein of the virus. The mode of packing of the nucleoprotein within the virion is still obscure.