Rearing Porthetria dispar (L.), through 1 generation at light regimes of 11, 13, 15, 17, and 19 hours per 24-hour day at temperatures of 19.5 and 28°C had no influence on induction or termination of diapause. Rearing temperature of 28°C caused egg mortality. A few eggs obtained in rearings at 19.5°C hatched without chilling. However, this hatch occurred 14-18 weeks after oviposition. This hatch of unchilled eggs appears to be analogous to the frequent observations in nature of egg hatch in the fall. Treatment of eggs up to 24 hours old with hydrochloric acid did not affect induction of diapause.