Efficacy of the suspension concentrate formulation of deltamethrin (Centynal) was evaluated at labeled rates on concrete surfaces and hard red winter wheat against 12 field strains of the red flour beetle, Tribolium castaneum (Herbst); six strains of the sawtoothed grain beetle, Oryzaephilus surinamensis (L.); and four strains of the lesser grain borer, Rhyzopertha dominica (F). The time for ≈100% mortality of adults of laboratory strains was first established by exposing them to deltamethrin-treated concrete at the highest labeled rate of 0.02 g(AI)/m2 for 1–24 h. On concrete, deltamethrin did not provide complete mortality of adults of T. castaneum and O. surinamensis field strains, but was effective against R. dominica strains. Mortalities of two strains each of T. castaneum and O. surinamensis were significantly lower than that of the corresponding laboratory strains. On wheat treated with 0.5 mg(AI)/kg of deltamethrin, the 7- and 14-d mortalities of four to six T. castaneum field strains, three O. surinamensis strains, and one R. dominica strain were significantly lower than that of the corresponding laboratory strains. Even at twice the labeled rate on wheat, the 7-d mortalities of three strains each of T. castaneum and O. surinamensis were significantly lower than the corresponding laboratory strains. Except for one T. castaneum and two O. surinamensis strains, the reduction in adult progeny production on deltamethrin-treated wheat relative to that on untreated wheat was 92–100%. Reduced susceptibility in field strains may be due to low levels of tolerance or resistance to deltamethrin.