SUMMARYA detailed examination has been made of the natural infection in two field experiments with sweet peas. Common pea mosaic virus (CPMV) and pea enation mosaic virus (PEMV) accounted for infection in most of the plants.In the first experiment, evidence was found of differences in ‘field resistance’ of fifteen varieties to PEMV but not to CPMV.The second experiment indicated that, though soil application of granular systemic insecticides rendered the plants toxic to aphids, it did not significantly reduce the spread of CPMV. The insecticides had no effects on the growth or flowering of the plants.Incidence of CPMV infection was significantly correlated with trap catches ofAcyrthosiphon pisum, Myzus persicaeandAphis fabaeand PEMV infection withAcyrthosiphon pisumandMyzus persicae.CPMV was not transmitted mechanically by flower cutting but the rate of infection was increased when the plants were layered.The epidemiology of CPMV and PEMV in relation to sweet peas is discus