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A mesophilic strain of Pseudomonas aeruginosa, grown at 30" and suspended to a concentration of equiv. 0.02 mg. dry wt/ml. in 30 mM-NaCl, showed rapid loss of viability when chilled rapidly from 30" to 0" or -2". The viability of a psychrophilic pseudomonad, grown at 30", did not decrease to the same extent when a dilute suspenision (equiv. 0.02 mg. dry wtlml.) was rapidly chilled from 30" to 0" or -2". Concentrated suspensions (equiv. 3.0 mg. dry wtlml.) of both the mesophile and the psychrophile released about the same proportion of total endogenous ultraviolet-absorbing compounds when rapidly chilled from ,30° to 5", oo or -2". Loss of viability following rapid chilling of a dilute suspension of the mesophile or the psychrophile was partly or completely prevented by 5 mM-Mg2+ and by bacteria-free filtrates from chilled concentrated suspensions of either bacterium. The viability of the bacteri,a grown at 10" did not decrease when dilute suspensions were rapidly chilled from 10" to -2". Bacteria grown at 10" contained a greater proportion of unsaturated fatty acids than bacteria grown at 30". Fatty acid analyses showed that susceptibility of the mesophile and psychrophile to cold shock could be correlated with the contents of unsaturated fatty acids in their lipids.

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