To evaluate the pesticide resistance of two commercial strains of Phytoseiulus persimilis Athias Henriot (Koppert, Strain A; Applied Bio-Nomics, strain B) and two strains of Amblyseius fallacis (Garman) (resistant and susceptible), residual toxicity trials were conducted on greenhouse cucumber crops that were sprayed with two pesticides (pyridabern and dicofol), respectively. Strain A had significantly higher mortalities than strain B when exposed to foliage residues of pyridaben and dicofol. Strain B showed 3-18% higher survivorship than strain A when exposed to 7-to 14-day foliage residues of pyridaben at 213 and 426 ppm. When exposed to 1-and 8-day foliage residues of dicofol at 152 and 303 ppm, strain A exhibited 8-10% higher mortalities as compared to strain B. Residual toxicity of both pesticides to P. persimilis decreased significantly at the lower concentrations and over time under greenhouse conditions. Mortalities of both strains of P. persimilis were reduced by 50% when exposed to the 1-seek residues of both pesticides at the lowest concentrations. With the treatment of dicofol (303 ppm), mortalities for the two strains of P. Persimilis decreased from 53.54-42.68% at 1-day residues to 28.18-21.21% at 8-day residues. At 500 ppm of dicofol, mortalities decreased from 60.09-60.90% at 1-day residue to 35.75-34.14% at 22-day residues. However, with dicofol (1000 ppm), mortalities were as high as 61.73% at 22-day residue. The resistant strain of A. fallacies showed significantly lower mortality, as compared to the susceptible strain, when exposed to 1-and 7-day foliage residue of pyridaben at 213 and 426 ppm concentrations. Mortality of A. fallacis to pyridaben was lower than that of P. persimilis. This suggests that A. fallacis may be more compatible with the application of this pesticide to greenhouse crops than that of P. persimilis. In addition, strain B of P. persimilis may be more advantageous after the application of these pesticides in a greenhouse IPM program than strain A.