A granulosis virus was tested on a field scale at Manjimup and Pemberton in Western Australia for control of its host, the potato moth Phthorimaea operculella (Zell.). The virus was produced on laboratory-reared larvae, and applied as a suspension of pulverised larvae (6 275 in 111 gal/ha) to second-crop (December-planted) potatoes. There were untreated and insecticide-treated (DDT, dieldrin, methyl-demeton) fields for comparison. The effectiveness of the treatments was assessed on numbers of larval mines and tuber damage. Virus infection rates of 100% were achieved, with a residual effect of 12 weeks. Variation in larval body weight indicated that initial dosages and those due to recycled virus were excessive, but were optimum after two months. Virus applications were as effective in preventing damage as insecticide treatments. Virus spread to untreated crops some miles distant was attributed to movements of personnel, farm materials or birds.