Weissbach, Arthur (National Institutes of Health, Bethesda, Md.), Allan Lipton, and Arnold Lisio. Intracellular forms of λ deoxyribonucleic acid in Escherichia coli infected with clear or virulent mutants of bacteriophage λ. J. Bacteriol. 91:1489–1493. 1966.—Infection of either the sensitive or lysogenic strain of Escherichia coli K-112S by λ+ leads to the formation of a new phage deoxyribonucleic acid (DNA) species having the properties of a twisted circular DNA duplex. This new phage DNA species is also seen in cells infected with clear or virulent mutants of λ which cannot lysogenize, or do so at a low frequency. The sedimentation rate of circular λ DNA duplex at various pH values and its lability were examined.