Potato leafroll luteovirus (PLRV) is transmitted naturally by aphids, but two isolates (15 and V) are known to be only poorly transmissible (PAT); these isolates are also distinct in that their particles lack an epitope present in transmissible (HAT) isolates of PLRV (Tamada et al., Ann. Appl. Biol. 104, 107-116, 1984; Massalski and Harrison, J. Gen. Virol. 68, 1813-1821, 1987). Virus cultures propagated vegetatively for several years in potato plants from the source of isolate V were shown still to be poorly transmissible by Myzus persicae. Moreover, when isolate V was transmitted by aphids, the subisolates obtained (V4 and V31) were no more readily transmissible than was the stock culture PLRV-V. The subisolates were also unchanged antigenically by the transmission. Thus the inefficient transmission had not restored the properties of HAT isolates. Sequence comparisons among the coat proteins and the readthrough domains of HAT and PAT Scottish isolates suggested that either or both of two amino acid changes in the C-terminal part of the readthrough protein are responsible for the poor transmission and the loss of an epitope. The readthrough protein is therefore thought to play a role in the circulative aphid transmission of PLRV.