Adult females of L. elongatus transmitted Glendevon TBRV less often than did larvae. Transmission was obtained with nematodes extracted through the Baermann funnels as efficiently as with nematodes directly hand-picked from screened samples. The level of transmission was somewhat variable, due to various factors, not yet fully known. Using a number of TBRV isolates, experiments supported the idea of specificity of transmission as suggested by Harrison in 1964. Thus only isolates from eastern Scotland, serologically indistinguishable from each other, were transmitted by Glendevon L. elongitus. Conversely, isolates from Germany and England, serologically distinct from the Scottish isolates, were not transmitted. However, these foreign isolates, although not transmitted, were recovered directly from the nematodes. The same was also found with isolates of raspberry ringspot virus (RRV). The nature of predominance of RRV in field outbreaks of both TBRV and RRV was investigated in glass-house tests. The viruses and L. elongatus came from the same source. Single larvae of L. elongatus acquired both viruses but the role of such viruses in transmission was unknown. RRV became dominant over TBRV when Petunia plants were inoculated with both viruses simultaneously. It seemed likely, therefore, that interference between the two viruses in plants may explain the predominance of RRV in field outbreaks.