Four insect species inhabit ball galls on goldenrod plants (Solidago sp.) for several months of the year. These sites are highly exposed and conceivably undergo wide fluctuations in temperature on a daily basis. I recorded temperature profiles for galls in western New York between September and November. The capacities of a gall inhabitant (Eurosta solidaginis larvae) to endure high temperature and freezing were also recorded during this period. I noted considerable and concurrent variation in air temperature and gall temperature. Diurnal radiant heating augmented changes in gall temperature so that their daily variations often ranged between 25 and 29 °C. From September to November, Eurosta larvae tolerated 24-h exposures to 35.5 °C and freezing at −6.5 °C. In the latter case, this occurred despite the lack of any appreciable accumulation of cryoprotectants in September and October. Greater freeze tolerance was gained by November (100% survival at −15.5 °C), when hemolymph osmolality increased 60% to 722 mosmol. This study demonstrates the extremely eurythermal nature of ball galls and the highly developed capacities of gall inhabitants to survive extreme temperatures.