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Abstract. 1. Host races, i.e. host-adapted populations in partial reproductive isolation, have been reported in a growing number of insect herbivores including aphids. The pea aphid Acyrthosiphon pisum encompasses distinct races highly specialised on various Fabaceae plants. Races have been well documented at both the ecological and genetic level. However, little is known about possible behavioural differences between host races other than those involved in plant choice.2. The aim of this study was to determine whether pea aphid host races differ in the behavioural response to the aphid alarm pheromone that was used to simulate immediate predation risk. The behaviour of aphids towards a synthetic alarm pheromone applied in their vicinity was measured in several pea aphid clones originating from various legumes including Medicago sativa, Pisum sativum, Trifolium pratense, and Vicia faba. To control for the influence of plant traits, experiments were conducted on the preferred host plant and on the universal host plant Vicia faba.3. Host races differed in their defensive responses, with stronger behavioural responses in host races originating from M. sativa and T. pratense compared with those coming from P. sativum and V. faba. These differences in behaviour are inherent to the pea aphid host race but not dependent on the test plant.4. Our work shows that host-plant specialisation may involve behavioural traits in addition to host choice. Possible mechanisms of how different defensive behaviours might have evolved are discussed.