OBPs have been recently demonstrated to be required for odour perception in insects and directly involved in odour discrimination. In aphids they might represent new interesting targets for the control of their population in agriculture. Based on sequence information available in the EST database, we have cloned four genes encoding odorant-binding proteins (OBP) in Acyrthosiphon pisum and homologous genes in other aphid species. Unlike OBPs from other orders of insects, that are greatly divergent, in aphids these proteins have been found to be highly conserved, with differences between species limited to only few amino acid substitutions. On the contrary, similarities between OBP sequences of the same species are poor with 31% or less of identical amino acids. Three selected OBPs (OBP1, OBP3 and OBP8) have been expressed in bacteria and purified. Ligand-binding experiments have shown similar behaviour of the three proteins towards several organic compounds, but also some significant selectivities. In particular, (E)-beta-farnesene, the alarm pheromone and its related compound farnesol exhibited good affinity to OBP3, but did not bind the other two proteins. We suggest that OBP3 could mediate response of aphids to the alarm pheromone.