The structure of intracellular DNA extracted from phage T1 infected cells was analysed by sedimentation through sucrose gradients. DNA labelled with 3H-dThd during a short pulse given at any time during T1 DNA synthesis sedimented in neutral gradients as a broad heterogeneous band with a large fraction of the label sedimenting more rapidly than mature T1 DNA molecules. Rapidly-sedimenting label was also observed when pulse-labelled DNA was denatured and analysed on alkaline sucrose gradients. Electron microscopy of intracellular T1 DNA revealed linear molecules of variable length the longest of which were three to four times the mature genome length. The distribution of lengths derived from electron microscopy are consistent with the molecular length distributions calculated from the sedimentation coefficients. We conclude that the rapidly-sedimenting DNA is in the form of concatemers consisting of linear tandem repeats of the T1 genome. The concatemeric form of replicating T1 DNA is a precursor of progeny T1 genomes since in pulse-chase experiments it was converted efficiently into mature, infectious T1 phage particles. The identification of this concatemeric form of T1 DNA provides supporting evidence for the model proposed by Gill & MacHattie (1976) to account for the formation of the very limited number of cyclic permutations of gene sequence found for mature T1 DNA molecules.