When passaged at high multiplicity, four strains of Sendai virus all showed evidence that they contained defective interfering (DI) particles. RNA isolated from nucleocapsids of cells infected with the high multiplicity passage stocks was found to consist of only minor amounts of nondefective genome length RNA and major amounts of smaller RNAs, the DI-RNAs. These DI-RNAs were found to have unusual and variable sedimentation properties in sucrose gradients, but were found to represent unique segments of the viral genome by length measurements in the electron microscope and by hybridization. A striking feature of the DI-RNAs is their ability to form circular structures, indicating that the ends of the DI-RNA are complementary. The implications of this finding in terms of the mechanism of genome replication is discussed.