The polyhedral virus disease of the wattle bagworm is described together with a method of purifying and assaying the virus. A procedure is suggested for making preparations of the virus suitable for spraying large areas.Aqueous suspensions of 10,000 polyhedra/cu. mm. applied to trees when the larvae were hatching caused a very high mortality; there was no increased mortality when higher concentrations were used. Newly hatched larvae showed symptoms after 3–4 days' feeding on contaminated food and they died 2 days later. Older larvae died more slowly, but the mortality was still high enough to prevent serious defoliation of the infected trees.The polyhedra remained infective out‐of‐doors for long enough to suggest that a high mortality could be obtained by spraying before the larvae emerged. Once disseminated in the field, polyhedra were not washed off by heavy rain and they appeared not to affect insect parasites of the ba