none. Field experiments were conducted to study the influence of cabbage monoculture and mixed cropping on the parasitism of diamondback moth, Plutella xylostella (L.), a destructive pest of all crucifers, by 2 larval parasites, Diadegma semiclausum Hellen and Cotesia plutellae Kurdjumov. There was no significant difference in parasitism by either species whether cabbage was planted in insecticide-free monoculture or in mixed cropping with 8 noncrucifers which were sprayed twice a week with chemical insecticides mevinphos, methamidophos and permethrin. Population of P. xylostella increased as the cabbage plants grew older. Parasitism by C. plutellae was higher soon after cabbage transplanting but decreased as the plants grew older. Parasitism by D. semiclausum was very low soon after cabbage planting but increased as the plants grew older. A significant negative correlation was found between parasitism by C. plutellae and D. semiclausum. In a caged field study where only one parasite species was used in an individual cage, parasitism of P. xylostella by both species decreased as the P. xylostella population increased. This is believed to be due to the absence of competition between the two parasites inside the cage. There was no relationship between host-plant age and parasitism of P. xylostella larvae by either parasite species. Shanhua, Taiwan.