Abstract: The black bean aphid (Aphis fabae) was heavily parasitized by the aphid parasitoid, Lysiphlebus fabarum both on agricultural crops (broad bean, fodder beet) and on associated weeds (goosefoot, chamomile, tansy). Parasitization rates increased during the season and were lower on broad bean than on other host plants. Habitat diversity contributed to an increase of parasitism on fodder beet but not on broad bean. Ant-attendance lead to increased parasitism on all host plants. The findings of the field studies were confirmed by laboratory experiments on foraging and oviposition behaviour of L. fabarum females on the various host plants.