Populations of the spotted tentiform leafminer, Phyllonorycter blancardella (Fabr.), from sprayed orchards across southern Ontario were found highly resistant (about 160-fold at LC50) to azinphosmethyl. Cross resistance to phosmet and partial cross resistance to diazinon were also indicated. All populations were highly susceptible to several synthetic pyrethroid insecticides. Resistant populations were controlled with pyrethroid insecticides applied when eggs were first found or by methomyl applied at about 50% hatch.