The aim of the present study was to compare ascorbate (ASA) content and ascorbate peroxidase (APX) activity in seedlings of the less susceptible triticale cultivar Witon and the more susceptible triticale cultivar Tornado after infestation by the grain aphid Sitobion avenae (F). and the bird cherry-oat aphid Rhopalosiphum padi (L.). The content of ASA in triticale seedlings slightly decreased after 24 hrs of infestation by cereal aphids, but prolonged feeding (48 hrs and 72 hrs) resulted in significant losses in ASA dependent on cultivar and aphid species. The aphid infestation caused an increase in APX activity in triticale tissues throughout the test period. The aphid herbivores induced APX activity to a higher level in a less susceptible cultivar than in a more susceptible one. The oligophagous species R. padi caused a higher decrease of ASA and stronger induction of APX activity in tested triticale than the monophagous species S. avenae. The experiments carried out indicate that ascorbate and ascorbate peroxidase may play a significant role in the defense mechanism of aphid infested triticale.