Within heavily defoliated areas, gypsy moth,Lymantria dispar(L.), larvae develop 1–3 wk faster than they do in lightly defoliated sites. On sunny days, we found that larvae in an outbreak population were 2–6°C warmer than those in a nearby low-density population. This temperature difference appears to result largely from density-related shifts in the microhabitat of the larvae. Data from a subsequent laboratory rearing study indicate that this temperature difference is sufficient to account for the relatively rapid larval development that occurs during outbreaks.