The genome of Choristoneura fumiferana nuclear polyhedrosis virus (CfMNPV) contained reiterated sequences interdispersed in four locations. These regions, termed RS, were found in EcoRI fragments A, F, E and B. The sequences were identified by hybridization of the fragment EcoRI-A to a Southern blot of EcoRI-digested viral DNA. Further confirmation and more precise localization of the RS sequences was obtained by hybridization of nick-translated 32P-labeled EcoRI-E fragment to Southern blots of viral DNA digested with EcoRI, BamHI, XbaI and Bg/II. Hybridization of 32P-labeled EcoRI-E to HindIII blots of viral DNA revealed the presence of a 'ladder' consisting of eight fragments. The three fragments of the ladder with the lowest sizes represented the HindIII fragments, O, PQ and R. The other five fragments were submolar in amount, in that they could not be seen in ethidium bromide-stained gels and probably represented minor virus variants that arose after passage of virus in larvae. Each variant was distinguished from the others by an additional insertion of 210 bp into the EcoRI-B fragment of the genome.