Abstract A method for the rapid diagnosis of resistance to malathion in adult Plodia interpunctella (Hübner) is described. Moths were exposed to a dry film of malathion coating conical flasks and knockdown was used as the criterion of response. A pooter to minimise damage to moths during handling is described. From probit mortality lines a discriminating dose killing all moths of susceptible strains was derived. Of 15 strains of P. interpunctella tested, resistance to malathion was detected in strains from Kenya, Nigeria and the U.S.A. and in strains imported into the U.K. on foodstuffs originating from Argentina, the Republic of South Africa and the U.S.A.