Phosphine-resistant populations of lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) and red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) were reported in Oklahoma, U.S.A. in 2012. Knowledge of the fitness effects associated with phosphine resistance is important for the development of resistance management strategies. Therefore, the goal of our study was to determine if there were fitness effects associated with phosphine resistance in populations of R. dominica and T. castaneum from Oklahoma. We measured population growth and developmental rates of phosphine-resistant and -susceptible populations of these two species in a phosphine-free environment. Three resistant R. dominica populations exhibited lower population growth and developmental rates compared to the susceptible population, whereas the only resistant T. castaneum population tested exhibited higher population growth and developmental rates compared to the susceptible population. Our data indicate that there is a fitness cost and possibly a fitness benefit, respectively, associated with phosphine resistance genes in these two species. In conclusion, phosphine resistance development in susceptible R. dominica populations can presumably be slowed by infrequent use of phosphine, whereas it can be mitigated by suspending phosphine use for extended periods of time in resistant populations. However, withholding phosphine use for long periods of time may not mitigate phosphine resistance in T. castaneum.