Phosphine gas (PH₃) is one of the most commonly used fumigants for controlling stored-grain pests worldwide. We estimated the discriminating dose for Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae) adult insects using a laboratory susceptible strain. This discriminating dose was then used to determine presence or absence of PH₃ resistance (resistance frequencies) in 19 field-collected populations of C. ferrugineus from Oklahoma, United States. The discriminating dose was estimated as 56.2 ppm of PH₃ over a 20-h exposure period at 25 °C. Discriminating dose bioassay results showed that PH₃ resistance was present in all 19 populations of C. ferrugineus tested. However, five populations—Stillwater (Stil), Enid Terminal 1 (ET-1), Enid Terminal 2 (ET-2), Johnson-Enid population (JE), and DK Farm 20 population (DK Farm 20) had ≥90% resistance frequencies. LC₉₉ values estimated by probit analyses of dose–response mortality data for the laboratory susceptible strain (Lab-S), JE, and DK Farm 20 were 7.3, 636.4, and 968.6 ppm, respectively, over a 3-d exposure period. The level of resistance in DK Farm 20, the most resistant population, was 133.5 times that of the susceptible laboratory strain. This study shows that PH₃ resistance in C. ferrugineus may be widespread in Oklahoma. Based on this study, there is a need for a wider PH₃ resistance survey in grain-growing regions of Oklahoma and United States. Furthermore, results show there is a need to develop PH₃ resistance management strategies for C. ferrugineus and other stored-product insect pest species to combat resistance and ensure continued effective future use of PH₃.