Large-scale field experiments on the Eastern Shore of Virginia during 1960 indicated that a nuclear polyhedrosis virus could be artificially disseminated either alone or in insecticide sprays to commercially control the cabbage looper, Trichoplusia ni (Hubner). Weekly applications of virus in the insecticide sprays in commercial plantings maintained the looper populations at an extremely low level throughout the growing season. Intensities of looper population resulting from natural virus infections on untreated broccoli at Painter, Virginia, demonstrated a cyclic behavior that was not observed at College Park, Maryland. At College Park the nuclear polyhedrosis virus is believed to have naturally occurred early in the season while looper populations were at low level and the population remained low throughout the season, June through October. These observations indicate that frequency of treatment by the virus for commercial control may vary in different areas.