Surface and internal temperatures of bags made by Thyridopteryx ephemeraeformis (Haworth) as well as ambient temperatures were recorded in sun and shade. Temperatures of bags in both shaded and exposed areas of the tree were significantly higher than ambient air temperatures. Bag surface temperatures were not significantly different from internal bag temperatures. Differences between internal, surface, and ambient temperatures could not be attributed to host tree species. Finally, there were no significant differences in temperature between bags with and without larvae.