Add to ORKGEggs of 12 species of sod webworm were exposed for various periods to temperatures of 0, 10, and 45°C. Hatchability was compared with control eggs kept at 25°C. Short exposures to any temperature did not significantly affect percent hatch. Prolonged exposure resulted in a significant decrease in hatchability for several species at 0° and for all species at 45°C. There was a significant difference in hatchability between the 3 generations of Crambus teterrellus (Zincken) and of Pediasia trisecta (Walker) at extreme temperatures. Populations of Agriphila ruricolella (Zeller), collected at 2 elevations, differed significantly in hatching percentage and developmental rate