Cauliflower mosaic virus (CaMV) aphid transmission factor (ATF or P18) is presumed to interact with both virus particles and vector mouthparts, thereby mediating virus aphid transmission. We developed a protein-protein binding assay and our results clearly show that virus particles bind strongly and specifically to P18 whether P18 was obtained from plants, a baculovirus expression system, or the pGEX-3X Escherichia coli expression system. We overproduced, using the pGEX-3X expression system, various fragments of P18 and thereby demonstrated that the C-terminal 31 amino acid residues are responsible for the interaction. Using PCR-based mutagenesis, 2 amino acid residues essential for interaction were identified. Point substitutions (amino acids 157 from Ile to Asn or 159 from Gly to Ser) were sufficient to abolish the interaction, whereas another mutation (amino acid 158 from Ile to Ser) had no effect on P18 virus binding. We evaluated whether there was a correlation between the ability of P18 to interact with CaMV particles and its biological activity. Aphid transmission assays were carried out and we demonstrated that the loss of the virus binding capacity had a dramatic effect on the ability of P18 to mediate aphid transmission. Thus, our results suggest that binding between P18 and virus particles is likely to be one of the molecular mechanisms involved in CaMV aphid transmission.