The lesser mealworm, Alphitobius diaperinus, is an invasive tenebrionid beetle infesting poultry facilities worldwide. As large population outbreaks occur regularly in broiler farms, insecticides are massively sprayed to control populations in poultry houses. In some countries, there has been evidence that A. diaperinus can develop resistance to several classes of insecticides. Here, we evaluated the insecticide susceptibility of several A. diaperinus populations collected in 2018 from various poultry farms in Northern Brittany (France). The adults were exposed to increasing doses of four different insecticides (technical-grade): two pyrethroids (ß-cyfluthrin, permethrin), and two organophosphates (azamethiphos and pirimiphos-methyl). Results revealed the existence of significant resistance to ß-cyfluthrin in three out of 11 populations. One population in particular was extremely resistant to ß-cyfluthrin. For this population the highest tested dose, equivalent to 500 times the recommended application rate (i.e. 10 g pure ß-cyfluthrin per m²), was not even sufficient to induce a low level of mortality. In contrast, for permethrin, azamethiphos, or pirimiphos-methyl, the results from dose-response bioassays did not suggest the occurrence of resistance. Hence, insecticides containing ß-cyfluthrin as active ingredient, which has a long use history in France, should be avoided in poultry facilities.