Effects of volatile odors from leek, Allium porum L., on the behavior of bean ßy, Ophiomyia phaseoli (Tryon) (Diptera: Agromyzidae), were tested in laboratory olfactometer bioassays. Aqueous and solvent extracts (dichloromethane and methanol) of leek were repellent to adult ßies. Whole leek plants were repellent and prevented attraction to the host plant, beans. Beans that had been exposed to volatiles from living leek plants for 7 d became repellent to the ßy. Leek and several other crops were tested in Þeld experiments to identify candidate crops for a mixed cropping system to minimize bean ßy attack in beans, Phaseolus vulgaris L. In a wet season Þeld experiment, mixed cropping of bean with leek or three other vegetable crops did not signiÞcantly reduce bean ßy infestation or infection with Fusarium oxysporum Schltdl. compared with amonocrop, but signiÞcantly reduced plant death caused by both agents combined. In two dry season Þeld experiments, mixed cropping of beans with leek signiÞcantly reduced adult bean ßy settling, emergence, and death of bean plants compared with a mono crop. Bean yield per row was �150% higher for the mixed crop, and economic returns were approximately Sri Lankan Rs. 180,000/ha, higher than for the mono crop. For the mono crop, the farmer had a monetary loss, which would become a small proÞt only if the costs of family labor are excluded. The study is an example of the Þrst steps toward development of sustainable plant protection in a subsistence system.