The growth rates, the mutation frequency rate constants of the terramycin-resistant cells, the burst size of the phage-producing cells, and the ratio of phage to cells all have a temperature coefficient of about 2 from 20 to 35 degrees (micro = 9 x 10(3) calories), with a maximum at 40 degrees . The mutation frequency rate constant (or time rate constant) of the phage-producing cells increases from 20 to 45 degrees with a temperature coefficient of about 3 (micro = 2 to 3 x 10(4) cal.). The change in the values for the growth rate, mutation rate, and cell volume occurs in less than 1 hour, after the temperature is changed. The value for the burst size of phage-producing cells changes for 3 to 4 hours. Prolonged growth of megatherium 899 at 48 to 50 degrees results in the production of C + S phage, in place of T. Returning the culture to 25 degrees results in the production of small T phage.