Cotesla plutellae is an endoparasitoid of larvae of the Diamondback moth (DBM) Plutella xylostella. This parasitoid plays an important role in the suppression of the DBM population. the authors evaluated toxicity of commonly used insecticides to Cotesla plutellae. the results shown that the insecticides kill 78.95 -100 percent of C. plutellae larvae indirectly by killing the hosts. Mortality of the hosts occurred 3 days after treatment. Most insecticides were highly toxic to C. plutellae adults, except the less toxic Delfin WG. the authors tested the toxicity of the insecticides to the pupal stage of C. plutellae. There were no Significant differences of percentage emergence between the four pupal ages after treatment. Mortality of newly emerged adults (1 day old) from pupae treated with Padan 95SP and Regent 800WG were highest (25.56 percent and 22.94 percent, respectively). Mortality of newly emerged adults 3 and 4 day old pupae treated with insecticides was very low (10 percent). The use of Bacillus thuringlensis had fewer adverse effects on the parasitoids, particularly on the pupal stsge of the parasitoid.