Several underwound families of topoisomers of the plasmid pIB130 were irradiated with fission neutrons at 28 C in TE buffer. Form I DNA followed the expected exponential dependence on dose, and was greater for those DNA molecules characterized by larger linking numbers. The radiation sensitivity was found to be linearly related to average linking difference. We tentatively attribute this enhanced radiation sensitivity to the increased number of transient open states that closed DNA experiences with increasing superhelicity. Radioprotectants enhance decrease in the DNA linking number in the presence of topoisomerase I, suggests that some of the protection conferred may be due to decreases in DNA superhelicity.