In field trials at sites of an outbreak of arabis mosaic nepovirus (AMV) in England and of raspberry ringspot nepovirus (RRV) in Scotland, the results of exposure of some new raspberry cultivars to natural infection by these viruses showed discrepancies from those obtained in graft inoculation tests using AMV-Lib and RRV-S, the Scottish type isolates. In particular, cv. Glen Prosen, which is immune to AMV-Lib and RRV-S, was infected with AMV and RRV in the field trials. Studies on these and other field isolates of AMV and RRV showed that they differed from the type isolates in Rubus host range and in symptomatology in herbaceous hosts. Four isolates of RRV found infecting Rubus were distinguishable by spur formation in gel double-diffusion serological tests but 6 AMV isolates were indistinguishable by this method. Immunoelectrophoresis of virus particles did not distinguish the 6 AMV isolates, but isolates RRV-MX and RRV-T were distinguishable from RRV-S and the English type isolate, RRV-E. Like the 2 RRV type isolates, RRV-MX contained a single electrophoretic component, but it migrated much faster whereas RRV-T contained 2 components, 1 with a migration rate similar to that of RRV-MX and the other similar to that of the type isolate. PAGE of protein preparations from highly purified virus particles of RRV isolates E, S and MX detected a single polypeptide of estimated MW 54 x 103, 54 x 103, and 50 x 103 respectively, but that of isolate T contained 2 polypeptides of estimated MW 54 x 103 and 50 x 103. These data suggest that RRV-T is a mixture of 2 isolates. In lab. tests the nematode Xiphinema diversicaudatum transmitted 4 isolates of AMV efficiently whereas 2 populations of the nematode Longidorus elongatus were less efficient vectors of 4 RRV isolates. Neither vector species transmitted virus to any of 9 raspberry cultivars. The results are discussed in relation to the control of nepoviruses in raspberry and to the biology of these viruses.