Most studies on transmission of plant viruses by vectors are concerned with single virus infections. Indeed care is usually taken to ensure, as far as possible, the clonal purity of the virus being studied so as to simplify interpretation of experimental results. Viruses in the Potyvirus group have flexuous filamentous particles about 750 nm long, and are transmissible by aphids in the classical nonpersistent manner. The ability of the helper component from one Potyvirus to assist aphid transmission of another is also widely reported, although some specificity in the interaction is usually observed. Caulimoviruses have about 50 nm diameter isometric particles containing a double-stranded (ds) DNA of about 8 kbp and a single particle protein of 42 kDa. They induce the formation in infected cells of characteristic “currant bun” inclusion bodies which consist of a proteinaceous matrix in which the virus particles are embedded. The dependent viruses in the complexes described above are defective or vector-specific strains of transmissible viruses.