Studies were conducted on the sample size requirement and bioassay methods for monitoring insecticide resistance in the diamond back moth (DBM), Plutella xylostella L Three sample sizes of 50,100 and 150 larvae and three bioassay methods viz. leaf dip, larval dip and vial assay were tried under laboratory. A sample size of 50 larvae was found to be adequate for monitoring insecticide resistance in DBM. Slopes resulting from different bioassay methods did not differ significantly indicating usefulness of these bioassay methods for monitoring resistance. However, leaf dip method was found to be ideal for monitoring insecticide resistance,which was further confirmed by the field validation studies.