Abstract: Alphitobius diaperinus Panzer and Tenebrio molitor L. are two species of the family of Tenebrionidae, which can infest and cause major losses in various types of stored products. In this context and considering that the published data regarding the efficacy of aerial insecticides against these species are extremely low, the aim of the present study was the evaluation of the immediate and delayed effect of phosphine gas (PH3) on different life stages of A. diaperinus and T. molitor. Life stages of both species (eggs, small larvae, large larvae, pupae, and adults) were exposed for three days at two different concentrations (50 and 100 ppm) of phosphine. After this time, the immediate effect was observed, whereas delayed mortality was evaluated 7 and 14 days after the end of the exposure. In addition, adults of both species were exposed at 3000 ppm of phosphine by using the Detia Degesch Phosphine Tolerance Kit, based on short exposures (several minutes). For both tested species, the most tolerant life stage was the egg, as all the other life stages tested were dead by the exposure at 50 ppm. Ninety minutes of exposure at 3000 ppm were adequate to lead to 100 % delayed mortality of T. molitor 7 days after the exposure. In contrast, 53.3 % of the A. diaperinus adults that had been exposed were active after a 14-days post exposure period. LT99 was determined as 5.54 and 4.34 for A. diaperinus and T. molitor adults, respectively, after exposure at 3000 ppm. To our knowledge, this research is the first that provided data regarding the efficacy of phosphine for the control of T. molitor and adds new information on this issue regarding A. diaperinus.