The suitability as a host of Diadegma armillata (Gravenhorst) (Hymenoptera, Ichneumonidae) was determined for eight West-European representatives of the genus Yponomeuta Latreille (Lepidoptera, Yponomeutidae) by dissecting and rearing parasitized larvae under controlled conditions. The suitability of the various species differed considerably, and dissection showed that this was mainly due to the ability of the host species to encapsulate the parasitoid eggs. Encapsulation by Yponomeuta -species corresponded fairly well to the differences between the parasitoid complexes concerning D. armillata . Classifying the Yponomeuta -species into three categories according to their suitability showed a remarkable parallelism with the phylogenetic relationships as determined by allozyme comparison. none