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Data for the inactivation of three Escherichia coli mutants by energetic heavy ions are fitted by the track theory of a one-hit detector in an extended target mode. The respective E0’s are 46, 36.5, and 12.6 Gy for E. coli B, B/r, and Bs-1, and a0, the assumed target radius, is 0.5 μm for all three. The parameter E0, the D37 with γ rays, is measured directly, while a0 is fitted to the data. It is significant that neither a point target model nor calculations with a0= 0.2 and 1.0 μm fit the data. Our fitted target radius, a0, approximates the size of the bacterium. These results raise questions as to why the E. coli mutants are one-hit detectors, and concerning the differences in the E0’s in relation to a mechanistic interpretation of cell killing.