Summary Endamoeba histolytica cysts, NRS strain, were exposed to temperatures of 45° to 54.5°C. in a thermostatically-controlled water bath for varying time intervals. The cysts were suspended in a concentration of 2,000 per ml. in 15 to 35 ml. of distilled water at a pH varying from 6.5 to 7.0. The criterion for cyst destruction was the subsequent inability of heated cysts to establish growth in culture. Some estimate of the amount of cyst destruction by heating was obtained by a comparison with growth results of known numbers of unheated control cysts. Failure to obtain cultures in any of 5 tubes inoculated with 1,000 treated cysts each was taken as indicative of 100 per cent destruction. In experiments at 45°C., the time required for complete cyst destruction was variable, but in all instances was more than 2 hours. At temperatures of 46°C. to 47°C., all cysts were destroyed within 1 hour. At a temperature of 48°C., cyst destruction was complete by 30 minutes. At 49° to 50°C., complete cyst destruction was effected within 10 minutes. At 51°C., all cysts were destroyed within 5 minutes. At temperatures of 52°C. and 54.5°C., all cysts were destroyed within 1 minute. It was noted that, at the lower temperatures, the time required for complete destruction was considerably in excess of the time required for approximately 99 per cent destruction. At the higher temperatures, this lag was proportionately less marked.