The bean aphid, Megoura crassicauda Mordvilko, feeds selectively on plants belonging to the genus Vicia (Fabaceae). However, it never infests the tiny vetch, V. hirsuta (L.) Gray. The aphid appeared to discriminate between host and non-host plants by tasting specific chemicals during penetration of its stylet into the plant tissues. The aphid, after being stimulated by specific probing stimulants, deposited characteristic proteinous stylet sheaths through a parafilm membrane, which has one side in contact with an extract solution of Vicia angustifolia. However, an addition of a V. hirsuta extract to the medium strongly inhibited the salivary sheath formation. A specific probing deterrent was isolated from a V. hirsuta extract by monitoring the inhibitory effect, and identified as (E)-2-methyl-2-butene-1,4-diol 4-O-beta-d-glucopyranoside. A mixture of the glycoside and the stimulatory V. angustifolia fraction in the same equivalency found in plants significantly decreased the probing activity in M. crassicauda. Since the stylet insertion process is a crucial step for the aphid's settlement on a plant, the glycoside seems to act as an effective chemical barrier for V. hirsuta.