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According to the theory of Butts and Katz and the new radial dose distribution of Zhang et al., we have calculated inactivation cross sections for the heavy ion bombardment of E. Coli B/r and Bs-1 which w& in agreement with the measurements of Scliafer et made with ions from O to U at energies from 1.5 to 19.5 MeV/u. The data display "thindown", and the decrease in cross section with an increase in stopping power is accompanied by a decrease in energy of a bombarding ion. Following an earlier analysis of heavy ion bombardments of these E. Coli mutants with low atomic number ions at energies neighboring 10 MeV/u by Katz and Zachariah, it can be seen that these bacteria act as 1-hit detectors on the response of these bacteria to ions of lower atomic number and LET where thindown is not exhibited. The analysis of thindown in these bacteria is much clearer than that in the case of mammalian cells because of the relative simplicity of the structure of these bacteria as compared to the complexity of mammalian cell structure. The significance of studying the thindown of E. Coli is to get further understanding of the radiation action in biology.