Foilar persistence of NPV of Mythimna separata (Walker) was carriedout during kharif 1991 and 1992. NPV treated leaves to the larvae of M. separata resulted in 85.88 per cent larval mortality, while it was 52.37, 19.32, 4.01 and 0.85 per cent after one, two, three and four days of feeding of larvae. The NPV of M. separata could remain active only for 24 h. on foliage under field conditions. More the period of exposure of virus, the persistence period was limited for short period and the virus lost its infectivity completely within five days of treatment.