Resistance of the predacious mite, Amblyseius swirskii (Acari: Phytoseiidae) to dimethoate in Egyptian citrus orchards. Amblyseius swirskii Athias-Henriot was reported. Samples collected from the citrus orchards representing different production areas in Egypt. Predators were transferred to the laboratory and reared. Dosage mortality relationships were used to measure the susceptibility of A. swirskii to the citrus insecticide dimethoate in laboratory bioassays. In orchards with no or occasional use of organophosphates, the field rate (1.5%) caused high mortality. A. swirskii from populations exposed to frequent applications of dimethoate were resistant (up to 12 times fold) to the field rate. In a moderately resistant population malathion was three times as toxic as dimethoate and diazinon 68 times. The synthetic pyrethroid, deltamethrin was 108 times as toxic. Under field conditions, application of diazinon in this orchard greatly reduced A. swirskii populations for at least 6 months.