The predatory mite Kampimodromus aberrans (Oudemans) is a key biocontrol agent in vineyards in Italy and Southern Europe. Its susceptibility to common pesticides (e.g., organophosphates) has been considered an important factor in preventing successful biocontrol of phytophagous mites. Nevertheless, populations of K. aberrans apparently resistant to organophosphates (OPs) have been reported to occur in Northern Italian vineyards. The resistance of K. aberrans to fungicides (e.g., mancozeb) has been demonstrated in the laboratory in France, but little is known about the toxicity of insecticides towards K. aberrans. Of these pesticides, the OP chlorpyriphos is extensively used in viticulture to control lepidopterans and homopterans. The present study investigated the dose-response effect of chlorpyriphos in four K. aberrans strains characterized by different levels of exposure to OP insecticides in the past: from never to frequently exposed. Resistance to chlorpyriphos is demonstrated for populations collected from vineyards and apple orchards. Resistance factors exceeded 145,000× for the three strains collected in vineyards and orchard. LC(50) values for resistant strains were 1.85-6.83 times higher than the recommended field dose of chlorpyriphos for vineyards and orchards (525 mg a.i./l).