Persistence of the nuclear polyhedrosis virus (NPV) of Mythimna separata (Walker) in soil was studied. The virus when incorporated in black and red soils and kept under field conditions had a half life of 9 and 13 months while it was 12 and 13.20 months when the same soils were kept under laboratory conditions. A comparison of the LT₅₀ values showed that the virus lost its virulence more rapidly when the virus treated soil was kept out doors. The virus stored in a refrigerator showed only slight loss in original activity even after 24 months