The effects of plant density of broccoli (Brassica oleracea L. var. italica) on the abundance of diamondback moth (DBM), Plutella xylostella (L.), and its parasitoid, Diadegma insulare (Cresson), were studied. There were no significant differences in the number of small DBM larvae (first and second instars) per plant across treatments (three different plant densities and three different spacings between plants) and sampling dates. The number of large larvae (third and fourth instars) and D. insulare pupae per plant were significantly influenced by plant density, sampling dates and the interaction of these two factors. However, the number of DBM pupae was not significantly different among treatments. Percentage parasitism of DBM larvae by D. insulare per plant did not vary significantly with either plant density or sampling date. Percentage parasitism of DBM larvae (field and laboratory-reared) per plant in the upper or lower one-third of the plant canopy was not significantly different among treatments and was not influenced by the interaction of plant density and height. This suggests that the hostsearching ability of D. insulare is not constrained either by plant density or height. The secondary (female to male) sex ratio of D. insulare and the number of D. insulare adults caught per trap (a measure of diurnal flight activity) were not significantly influenced by plant density and sampling date. However, the total catches of adults (both sexes) were significantly higher in the higher than in the lower plantdensity. Based on our results, we suggest that broccoli plant density should be chosen only when proven to have a negative impact on DBM population abundance. none