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Laboratory tests were carried out to evaluate the efficacy of Mahogany seed oil against Rhyzopertha dominica on cowpea. The test was conducted under ambient laboratory conditions (28-35O c and 65-70% r.h.). Thirty adult insects of unsexed were bioassayed on 50g cowpea grain sample treated with the Mahogany seed oil (MSO) at five concentration rates: 1.0, 1.5, 2.0, 2.5 and 3.0 ml/kg. Mortality of exposed adults were assessed after 7, 14 and 21days exposure to treated grains. Number of progeny were assessed at 56 and 112 days after removal of parent insects. Significance difference (p<0.05) in adults mortality were noted between different concentration rates and exposure periods on treated grain. Increase in concentration rates and exposure period resulted in higher adult mortality and progeny suppression. After 14days exposure to the highest concentration rates, 100% adult mortality was achieved. Progeny production was considerably suppressed even with lowest concentration rates of 1.0ml/kg 1.2plusminus0.6 and 1.0plusminus0.5 was recorded when compared with untreated control with 91.4plusminus2.9 and 116.0plusminus5.1 after 56 and 112days of storage respectively. Similarly, the percentage of insect damage kernel and weight loss decreased with increase in concentration rates. At 3.0ml/kg no single damage and weight loss recorded as compared to 100% and 81.6% in the untreated control, respectively. Moreover, the result indicate that germination percentage was not affected by different treatment concentration when compared with the untreated control thus at 1.0ml/kg 64.0plusminus8.3% germinated grain was recorded, at 3.0 ml/kg 99.2plusminus0.5% was observed when compared with 7.0plusminus2.8% on untreated control.