A virus causing a wilt ofDatura stramoniumwas identified as a strain of henbane mosaic virus. It causes necrotic local lesions inNicotiana rustica, and local lesions are demonstrable in tobacco by staining with iodine. Some of the factors affecting its transmission byMyzus persicae(Sulz.) were studied quantitatively using these lesions.Infective aphids differed little in their ability to cause infection, and usually produced two or three lesions. The duration of the feeding puncture did not affect the number of infections and had little effect on the percentage of aphids becoming infective. Transmissible virus did not seem to be continually imbibed while aphids fed on infected plants, and there were indications that it was acquired immediately before aphids withdrew their stylets from the leaf. Aphids became infective when allowed to make feeding punctures into epidermis stripped from infected leaves.M. persicaetransmitted during feeding punctures as brief as 5–10 sec; the probability of single feeding punctures resulting in infection reached a maximum with those lasting from 20 to 30 sec, during which the stylets did not penetrate as far as the centre of the epidermal cell and little or no saliva appeared to be ejected.M. persicaedid not transmit the virus when its stylets were artificially wetted with infective sap.Periods of darkness before inoculation with datura wilt virus increased the susceptibility ofNicotiana rusticato infection by rubbing, but not to infection by aphid