Effect of the insect growth regulators (IGRs) altosid and altozar which have a juvenile hormonelike activity, were investigated against the mid-aged tritonymphs of the so called North Americanhouse- dust mite, Dermatophagoides farinae Hughes. When the tritonymphs were exposed to altosid mixed in the feeding medium at the rates of 0.000161, 0.00028, 0.0028, 0.0161 or 0.0322 ppm the percentages inhibition of adult formation were found to be 26.7, 43.0, 67.0, 70.0 or 75.5 %, respectively. Altozar offered in the feeding medium at the concentrations of 0.000163, 0.000326, 0.00326, 0.0163 or 0.0326 ppm caused 28.3, 53.3, 56.7, 61.7 or 71.7 % inhibition of the development to the adult stage, in respect. Treating the mid-aged tritonymphs with either altosid or altozar caused the prolongation of the median times needed for the development of the adult stage (DT50) as they were 6.5, 9.9, 10.5, 9.3 and 1o.8 days for the forementioned concentrations of altosid, in respect; and 6.1, 7.7, 8.6, 11.1 and 11.5 days for the forementioned tested concentrations of altozar, in respect; in contrast to 2.2. days only for the checks. All the previously mentioned results showed regression relationships among log-concentrations and obtained effects in probits.