Intracellular viral DNA from African green monkey cells (AGMK) infected with simian virus 40 (SV40) has been isolated by repeated sedimentation through alkaline sucrose gradients. Four fractions with different sedimentation coefficients have been obtained and these viral DNA preparations have been analyzed by electron microscopy. The occurrence of circular oligomers and catenated SV40 DNA molecules has been demonstrated. Circular molecules between two and six times larger than SV40 monomers have been observed. Each different class of oligomeric DNA was infectious. The specific infectivity of these complex DNA molecules was in the range expected for viral DNA of this size.