Superhelical tension of DNA in living bacteria is believed to be partially constrained by interaction with proteins. Yet DNA topology is a significant factor in a number of genetic functions and is apparently affected by both genetic and environmental influences. We have employed a technique that allows us to estimate the level of unconstrained superhelical tension inside the cell. We study the formation of cruciform structures by alternating adenine-thymine sequences in plasmid DNA by in situ chemical probing. This structural transition is driven by superhelical torsion in the DNA and thus reports directly on the level of such tension in the cellular DNA. We observe that the effect of osmotic shock is an elevation of superhelical tension; quantitative comparison with changes in plasmid linking number indicates that the alteration in DNA topology is all unconstrained. We also show that the synthesis of defective topoisomerase leads to increased superhelical tension in plasmid DNA. These experiments demonstrate that the effect of environmental and genetic influences is felt directly at the level of torsional stress in the cellular DNA.