Pseudonionas aeruginosa grown in simple salts media in a chemostat showed vasying sensitivity to lysis by polymyxin and EDTA. The sensitivity to these agents depended both upon the growth limiting nutrient and the growth rate. Carbon limited bacteria showed no change in the high sensitivity to polymyxin with variation in growth rate (dilution rates 0-05–0-6 h−1). Polymyxin sensitivity of magnesium limited cultures was also independent of growth rate although these bacteria were approxi mately 6 times more resistant to this agent than carbon limited ones. EDTA lysis of carbon limited cells was shown to be related to growth rate, the cells being most sensitive at high growth rates. Magnesium limited cells were resistant to EDTA at all growth rates. The addition of barium to carbon or magnesium limited cultures was without effect. Calcium added to carbon limited cultures rendered the cells sensitive to EDTA at all growth rates but did not alter sensitivity to polymyxin. The addition of calcium to magnesium limited cultures altered the sensitivity pattern of these cells to EDTA, the slowest growing cells becoming the most sensitive. This cell type was also more sensitive to polymyxin.