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A mesophilic bacteriophage (K-20) first isolated on Bacillus pumilus W43 has been used to investigate the nature of exogenously conferred thermostability. The capacity of the suspending medium to protect K-20 against thermal inactivation at 65°c has been determined for a number of neutral salts at selected concentrations. Two distinct salt effects (electrostatic and lyotropic) are identified according to the concentration at which a given salt confers maximum protection. Two different nonlinear thermal inactivation curves of declining phage tit re plotted as the logarithm against time at constant temperature are discussed. A two-step, concave thermal inactivation curve for K-20 suspended in certain concentrated salts was investigated and interpreted as evidence of a “clumping” phenomenon. As unusual convex thermal inactivation curve observed for K-20 suspended in dilute divalent cation solutions is afforded a preliminary mathematical description. The occurrence of large proportions of atypically small plaques from phage which have been suspended and heated to a potentially lethal temperature in certain concentrated salt solutions is discussed. A mesophilic bacteriophage (K-20) first isolated on Bacillus pumilus W43 has been used to investigate the nature of exogenously conferred thermostability. The capacity of the suspending medium to protect K-20 against thermal inactivation at 65°c has been determined for a number of neutral salts at selected concentrations. Two distinct salt effects (electrostatic and lyotropic) are identified according to the concentration at which a given salt confers maximum protection. Two different nonlinear thermal inactivation curves of declining phage tit re plotted as the logarithm against time at constant temperature are discussed. A two-step, concave thermal inactivation curve for K-20 suspended in certain concentrated salts was investigated and interpreted as evidence of a “clumping” phenomenon. As unusual convex thermal inactivation curve observed for K-20 suspended in dilute divalent cation solutions is afforded a preliminary mathematical description. The occurrence of large proportions of atypically small plaques from phage which have been suspended and heated to a potentially lethal temperature in certain concentrated salt solutions is discussed. Thermal stability conferred on K-20 by T-broth, 10 x SSC, and Lincoln College tap water is discussed in the light of data for the protective influence of defined media.