The tritrophic interactions between two different plant-host complexes, Avena sativa-Sitobion avenae, Triticum aestivum-S. avenae and the aphid parasitoid Aphidius rhopalosiphi were studied with respect to odour learning and recognition by the parasitoid. The orientation behaviour of females towards odours from either uninfested or aphid-infested oat or wheat plants was tested in a series of dual choice Y-tube olfactometer experiments. Female parasitoids had the opportunity to gain a single oviposition experience on either the oat-S. avenae or wheat-S. avenae complex before the experiment. In the first set of experiments, where A. rhopalosiphi was reared on the oat-S. avenae complex, eight odour-bait combinations were tested. The females did not discriminate between uninfested oat and wheat. After oat complex experience, females responded to odours from the oat complex, but not to odours from the wheat complex. Consequently, in a direct comparison the oat complex was preferred over the wheat complex. After wheat complex experience, the parasitoid's orientation responses gave a different picture. Both, the wheat complex and the oat complex, were then shown to be equally attractive. Hence, in direct comparison no preference was recorded between the oat and wheat complexes. In a second set of experiments, where A. rhopalosiphi was reared on the wheat- S. avenae complex, a possible influence of any pre-adult or emergence-related host plant experience could be excluded as the same results were obtained as before. At first glance the responses towards the different odour baits seem inconsistent. However, the results may be explained using a simple model with two key odour components.