The study of various behavioural criteria of female Diadromus pulchellus parasitoids in the presence of their Acrolepiopsis assectella hosts has shown the essentially chemical nature of the stimulant determining host recognition. The physical stimuli of the cocoon seem not to be implicated. Thus cocoons whose original texture has been completely altered, either mechanically or chemically, as well as the silk excreted by the host caterpillars significantly stimulates the female parasitoids. The cocoon contact kairomones are detected in testing the aqueous extracts of A. assectella which provoke a positive behavioural response from the females in a threshold concentration of 1 cocoon-equivalent. The comparison of aqueous extracts of A. assectella host cocoons, and of non-host species: Bombyx mori, Ephestia kuehniella and Cacoecimorpha pronubana demonstrate the kairomone specificity of the silk, the extracts of non-host cocoons being ignored by the parasitoid, as were the silk threads left by E. kuehniella caterpillars. Finally the contact kairomones linked to the silk seem to be independent of the host plant and of the nutrient diet of the host caterpillars. The cocoons spun by the caterpillars reared on leeks or on artificial diets with or without powdered leek provoke similar responses in the parasitoids.