We tested the hypothesis that the entomopathogenic fungus Beauveria bassiana would be more efficacious on oat cultivars that prolonged the immature developmental period of sawtoothed grain beetle, Oryzaephilus surinamensis (L.), a storage pest. However, percentage of reduction in progeny production was similar on whole ‘Don’ and ‘Paul’ oats treated with fungus, even though immature developmental time was longer on whole ‘Don’ than on ‘Paul’ oats. In our initial test at 10 mg of conidia per kilogram of oats, the number of beetle progeny produced was reduced by 38–67% in whole oats, and there was no effect of the fungus on insects developing on cracked oats. Therefore, we conducted two dose–response studies that showed that adding 150 mg of conidia per kilogram to cracked or whole ‘Paul’ oats resulted in a 70 and 98% reduction, respectively, in number of progeny produced. No further reduction was obtained by adding 200 mg of conidia per kilogram of cracked or whole ‘Paul’ oats. Presence of the fungus did not affect developmental time in any of our tests. A previous study showed that cleaned oats should limit insect population growth to allow long-term storage of oats without insect damage. However, the current study shows that if the oats are not cleaned, and not cleaning oats is the normal storage practice, then B. bassiana could be used to help control sawtoothed grain beetles.