It has been previously demonstrated that photoreaction with 4,5',8-tri-methylpsoralen (trioxsalen) can be used as a probe for the in vivo structure of eucaryotic chromatin. We have used this probe to analyze the organization of intracellular Escherichia coli DNA. In contrast to eucaryotic DNA, bacterial DNA within the intact cell is not protected from saturating doses of trioxsalen photoaddition. The final level of covalently bound trioxsalen upon saturation is identical to that found with purified DNA. In addition, the distribution of interstrand DNA cross-links formed by low doses of trioxsalen photoadducts does not exhibit the repeating pattern that has been observed with eucaryotic nucleosomes.