The nuclear-polyhedrosis and granulosis viruses, the types of insect viruses being considered for development as microbial insecticides, are quite stable at moderate and low temperatures, may remain active for long periods in soil, are not affected directly by humidity, and are compatible with the majority of chemical insecticides. These viruses are readily inactivated by exposure to sunlight, strong acids or alkalis, and high temperatures. This discussion indicates that inactivation by sunlight is the most important factor causing loss of activity of viruses in the field environment.