Although new generation (prediapausing) adults of Dolycoris baccarum (L.) and Piezodorus lituratus (F.) (Heteroptera: Pentatomidae) spend the summer in fields, diapausing individuals spend the winter in overwintering localities under stones and plants. The phospholipid and triacylglycerol fatty acid composition of whole insects from prediapausing and diapausing adults was analyzed to determine changes in fatty acid unsaturation during diapause. The most abundant fatty acids from both fractions were palmitic (16:0), oleic (18:1), linoleic (18:2n-6), palmitoleic (16:1n-7), stearic (18:0), and linolenic (18:3n-3) acids. This represents a typical complement of heteropteran fatty acids. However, palmitoleic acid (16:1n-7) levels were higher in the triacylglycerol fraction in diapausing adults of P. lituratus (13.6%) than those reported from other heteropterans (1–3%) (Thompson 1973). The unsaturation/saturation ratio was significantly increased in triacylglycerol and phospholipid, apparently because of increases in major monounsaturated 18:1 at the expense of major saturated 16:0 during diapause. These changes were probably a response to low environmental temperatures.