This paper describes some characteristics of a granulosis virus specific to the larvae of potato moth (Phthorimaea operculella (Zell.)). Host eggs are laid on the soil or on the potato plant; the newly hatched larvae bore into the leaves, generally through the underside, and then develop in feeding tunnels. Mines occur more frequently on the lower leaves, and on the outermost and larger leaflets. The adults are dispersed by wind. Exposure of larvae to externally borne virus deposits is very brief, but sprays of the virus usually produce high rates of infection even when applied only to the upper surfaces of the leaves. It was established that virus particles can enter treated leaves through the stomata. Infected larvae die in two to three weeks, but an overdose causes death by toxicosis within 48 h. In two small-scale field trials the disease spread to control plots preventing comparison of damage to tubers of treated and untreated plants. In each trial, the sprays started an epizootic which affected almost all larvae on the treated plants. With high host populations and many volunteer plants, the epizootic spread at least 1 000 yd in five years. Under good commercial conditions with low host populations, wind and birds were the chief means of virus spread. The distribution and retention of virus particles on the plant were affected by wetting agents and virus concentration.