Persistence of the nuclear polybedrosis virus (NPV) of Spodoptera mauritia (Boisduval) in soil was studied. The virus when incorporated in soil and kept under field conditions had a half life of 6.46 months, while it was 16.03 months for the virus in soil 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 outdoors. The virus stored in a refrigerator showed only slight loss in original activity even after 18 months.