Summary 1) Rates of inactivation of tissue-culture-derived, foot-and-mouth disease virus, type A, strain 119 (FMDV–A119), at various pH levels and temperatures and by formaldehyde were determined. Ranges of pH and temperature investigated were 2.0 through 10.0 and 4°C through 61°C. respectively: formaldehyde was employed at 0.009%. The results are interpretable by first-order kinetics. However, at pH 5 and 6 and also at 55° and 61°C small fractions of the virus population had much lower first-order inactivation rates than the bulk of the virus. Possibilities concerning the nature of the fractions with higher resistance are discussed. 2) Rates of inactivation at various pH levels were determined at 4°C. Below pH 4 the virus was totally destroyed within a few seconds. At pH 5 and 6 infectivity was lost at a rate of about 90% per second and minute, respectively, until only one-millionth of the virus remained. This residual virus was very stable to further inactivation. At pH 6.5 and 10, 90% of the virus was inactivated every 14 hours. The virus showed marked stability only at pH 7 and 7.5. losing little infectivity during a 5-week period. At pH 8 and 9, a 90% reduction of infectivity occurred within a 3- and a 1-week period, respectively. 3) Rates of thermal inactivation were determined at pH 7.5. The time intervals required for the inactivation of 90% of the virus existing at any time were as follows: 18 weeks at 4°; 11 days at 20°; 21 hours at 37°; 7 hours at 43°; 1 hour at 49°; 20 seconds at 55° to a survival of 0.001, 7 minutes thereafter; and 3 seconds at 61°C to a survival of 0.00001, 11 minutes thereafter. Activation energies calculated for loss of infectivity below and above 43°C were 27.200 and 120,600 calories per mole of FMDV. respectively. 4) Virus treated with formaldehyde at a concentration of 0.009% was inactivated at a rate of 90% per day of storage at 4°C.