The susceptibilities (dosage-response regression curves) of northern fowl mites collected from six poultry farms in North Carolina were determined for five acaricides by exposure of the mites for 24 hr to acaricide residues inside glass pipettes. The residue concentrations were expressed as parts per million on a weight-volume basis of the acaricide-acetone solution in which the pipettes were immersed prior to testing. All of the acaricides, except malathion, were highly toxic to the northern fowl mites. The mean LD50 values (ppm) were: permethrin, .53; tetrachlorvinphos, 4.06; carbaryl, 4.11; and coumaphos, 5.04. For malathion, the mean LD50 for mites from three caged layer flocks was 119.35 ppm while concentrations as high as 500 ppm gave only low mortalities (20% or less) of mites from the other flocks and LD50 values could not be calculated. These data show that northern fowl mites are highly resistant to malathion in North Carolina, as has been reported elsewhere, but there is no evidence of resistance to the other acaricides tested. The effect of temperature on the toxicity to northern fowl mites of the newest acaricide, permethrin, was determined. Permethrin was significantly more toxic at 20 C than at 25, 30, or 35 C.