Temperatures were measured at the bark suface and in the subcortical zone of Pinus contorta DOUGLAS var. latifolia ENGLEM. infested by Dendroctonus ponderosae HOPKINS. and compared with similar measurements in non-infested trees and with air temperatures. Subcortical temperatures were higher at night but usually lower during the day than air temperatures. Surface bark temperatures were intermediate. Temperatures were lower at the 1-ft. level then at higher levels. Highest subcortical temperatures occured on the south aspect: daily maxima were later in the day and daily range was greater in in infested thanin non-infestd trees, especially during the late stages of beetle brood development. Thin-bark trees had higher temperatures and less temperature lag than thick-bark trees. Temperatures were 5.5 to 7.5 C higher under maritime tropical than maritime polar air. The effects of temperature on various aspects of beetle behavior, development and survival are dicussed. Approximately 8,340 degree-hours above 10 C were required in the subcortical zone for the beetle to develop from egg to tenerai adult.