A 10-fold difference in the LC50 response to phosmet insecticide was measured between 2 strains of a predatory mite, Amblyseius chilenensis (Dosse), collected from Uruguayan commercial apple orchards. Responses of these populations to single concentration levels of phosmet, azinphos-methyl, stirofos, parathion and carbaryl administered by a slide-dip and residue application method were variable indicating that certain resistance, cross-resistance or tolerance relationships existed among the 2 predator populations and the 5 insecticide treatments.