The use of petroleum oil prevented, to varying degrees: the transmission of three stylet-borne viruses in three different crops. The use of oils resulted in a marked improvement in the number of lily bulbs harvested from treated plots as compared to the number of lily bulbs harvested from untreated plots. A random sample of the harvested bulbs forced in the greenhouse showed that substantially more bulbs germinated from treated plots than from untreated plots. Lily virus symptoms were prevented in 13% of the plants in treated plots. All untreated plants were infected. However, this relatively small difference was probably due to the use of too many infector plants. Treated lilies showed more stunting and mottling than did the untreated lilies. The influences of oil sprays on the transmission of bean yellow mosaic virus in beans were studied in 1967 and 1968. Virus symptoms in 1967 were shown to vary significantly with planting dates. The early planting had a greater incidence of plants showing virus symptoms than did the later planting. Oil sprays reduced virus symptoms by 20.9% in the early planting and by 40.7% in the later planting. Oil sprays reduced BYMV symptoms from 10-30% in 1968, depending on the treatment. However, the statistical significance of these treatment effects was questionable. The spread of BYMV in 1968 was strongly correlated with the daily number of aphids trapped at two widely separated trap locations. These correlations were improved when the number of aphids trapped were adjusted for the possible effects of the oil sprays. The implications of the correlations were threefold. First, the direct relationship between virus symptoms and aphid numbers suggested that the timing of sprays to periods of high aphid migrations could improve virus prevention. Second, the prevention of virus spread could be improved by the application of oil during periods when one or more aphid species particularly efficient in virus transmission are migrating. Third, the value of oil could be improved by the maintenance of a highly efficient oil residue for longer periods of time. Oil sprays reduced white -break virus (cucumber mosaic virus) symptoms in gladiolus from 9.5% to 3.1% in the field. However, similar results were not obtained when sample corms from the field plots were forced in the greenhouse.