The DNAs of two isolates of strain CM4-184 of cauliflower mosaic virus, reported by other investigators to have deletions in different locations of the region II-region III portion of the genome, were found to give identical fragments when cleaved with BglII, BamHI, and HindIII restriction endonucleases. When these isolates of CM4-184 were cloned into the single-stranded vector M13mp7 followed by nucleotide sequence analyses, they had identical sequences. Comparison of the sequences of such CM4-184 deletion mutants with an undeleted strain of similar origin (strain CM1841) revealed that the CM4-184 deletion consists of 421 base pairs from open coding region II of the genome. Therefore region II, comprised of about 500 base pairs, is not essential for infectivity. It may represent a site at which foreign DNA could be inserted into the genome.