Inactivation of Pseudomonas sp. No. 128 was found to be independent of concentration up to a critical level of 2 × 108 org./ml. The highest sensitivity was noted in the exponentially-growing cultures, and the highest resistance in cultures in the negative accelerating growth-period (0·5 and 50·0 per cent survival, respectively, for 10·0 krad established immediately after irradiation). Resistance was essentially the same during lag and late declining periods (1·0 per cent survival for the standard dose used). Exponentially-growing cultures showed a continuously increasing inactivation on delay of the plating after irradiation (from 0·5 to 0·005 per cent survival at 10·0 krad). No such effects were noted for cultures in late declining or maximum stationary growth-periods. Survival-curves for samples taken from maximum stationary and declining growth-periods were exponential. A cumulative type of survival-curve was noted for organisms from the lag and logarithmic periods of growth. © 1963 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.