Raspberry leaf mottle virus is acquired by Amphorophora rubi in feeding periods of -2 hr. Longer feeding periods cause no significant fluctuations in infectivity. During continuous feeding on healthy plants, infectivity declines rapidly after 2 hr., but persists for at least 5 hr. if aphids are transferred frequently to fresh healthy plants. The estimated probability of infection by single aphids is 1 in 20. Rarely more than 50 % of test plants, Rubus occidentalis, seedlings became infected, and this is thought to be due mainly to variation in plant susceptibility. Young leaves of infected Norfolk Giant are, however, better sources of virus than older leaves.