none To assess the impact of air pollutants on the population dynamics of herbivores, the effects of pollutants on their natural enemies including predators, parasites, and pathogens must be evaluated in addition to direct effects and indirect effects mediated via the host plant. Insect parasitoids are an important group of such natural enemies providing many examples of partial or complete biological control of pest species. This study examined the effects of air pollutants (ozone (O-3), sulphur dioxide (SO-2), and nitrogen dioxide (NO-2)) on the searching behaviour of insect parasitoids. A series of experiments comprising short-term, closed chamber fumigations of O-3, SO-2, and NO-2 (100 nl 1-1) of the braconid parasitoid (Asobara tabida) and aggregated distributions of its host larvae (Drosophila subobscura) was set up. Analysis of chamber results showed that the proportion of hosts parasitised and the searching efficiency of the parasitoids were both significantly reduced with O-3 fumigation, but not with NO-2 or SO-2 fumigations. O-3 fumigation reduced percentage parasitism by approximately 10%. Parasitoids were able to avoid patches with no hosts, both in filtered air controls and when exposed to pollutants. However in the O-3 and NO-2 treatments they appeared less able to discriminate between different host densities, suggesting that pollutants may interfere with the olfactory responses of the parasitoids. These results indicate the potential for air pollutants, particularly O-3, to negatively influence the searching behaviour of parasitoids, and hence reduce the efficiency of natural enemy control of many pest species.