When polyhedral inclusion bodies (PIB) of the singly embedded nucleopolyhedrosis virus ofHeliothis, Baculovirus heliothis(HzSNPV), were exposed in water to simulated sunlight, there was about a 90% loss of its original viral activity, and the PIB were ≍3 times more sensitive than dry PIB. The pH (at 3,6, or 9) of the suspension had no significant effect on the extent of inactivation of PIB by simulated sunlight. Temperature of exposure (10, 22, 35, or 50%C) also had no effect on sensitivity of PIB exposed to simulated sunlight. Normally encountered field temperatures or pH, or both, coupled with exposure to sunlight, should not adversely affect the stability of PIB of HzSNPV. Presence of free water, however, can significantly increase the extent of PIB inactivation by sunlight.