Determinants have been mapped in the coat protein of cucumber mosaic virus (CMV) which influence transmission by the aphid Aphis gossypii. Chimeric RNA 3s were constructed which contain sequences from the efficiently aphid-transmitted strain Fny-CMV and the very poorly aphid-transmissible M-CMV. Analyses of chimeric viruses revealed that two regions in the coat protein were critical for transmission by aphids. The coat proteins of Fny- and M-CMV were previously shown to differ at eight amino acid positions. Amino acid changes were introduced into regions of the coat protein gene of M-CMV shown to be involved in transmission and two amino acid changes were demonstrated as essential for the restoration of aphid transmissibility. Nonconservative changes at positions 129 (Leu to Pro) and 162 (Thr to Ala) modified the coat protein to resemble Fny-CMV at these positions and restored transmissibility by A. gossypii. A change at amino acid position 168 (Cys to Tyr) may influence the efficiency of aphid transmission, but is not essential. A single amino acid change at position 129 (Leu to Pro) did not restore aphid transmissibility to M-CMV. The findings are discussed in relation to the nonpersistent transmission of plant viruses.