The cucumber mosaic virus (CMV) 2a RNA-dependent RNA polymerase protein has an additional function in Arabidopsis thaliana, which is to stimulate feeding deterrence (antixenosis) against aphids. Antixenosis is thought to increase the probability that aphids, after acquiring CMV particles from brief probes of an infected plant's epidermal cells, will be discouraged from settling and instead will spread inoculum to neighbouring plants. The amino acid sequences of 2a proteins encoded by a CMV strain that induces antixenosis in A. thaliana (Fny-CMV) and one that does not (LS-CMV) were compared to identify residues that might determine the triggering of antixenosis. These data were used to design reassortant viruses comprising Fny-CMV RNAs 1 and 3, and recombinant CMV RNA 2 molecules encoding chimeric 2a proteins containing sequences derived from LS-CMV and Fny-CMV. Antixenosis induction was detected by measuring the mean relative growth rate and fecundity of aphids (Myzus persicae) confined on infected and on mock-inoculated plants. An amino acid sequence determining antixenosis induction by CMV was found to reside between 2a protein residues 200 and 300. Subsequent mutant analysis delineated this to residue 237. We conjecture that the Fny-CMV 2a protein valine-237 plays some role in 2a protein-induced antixenosis.