Mortality of overwintering larvae of the wheat stem sawfly,Cephus cinctusNorton, was 92–93% in stubble exposed on the soil surface by tillage. Some larval mortality also occurred during April and May even though lethal low temperatures did not occur. There was 91-100% survival of larvae in untilled stubble. Field-collected larvae supercooled to −24.3°C in the laboratory. Larval mortality occurred before the supercooling point was reached. Fifty percent mortality occurred after 3 h at −22°C and 4–8 h at −20°C. There was no seasonal variation in larval resistance to freezing. Supercooling points were not significantly affected by plant material in larval guts.