Abstract: The effect of crude petroleum oil on invertase and amylase activities was determined directly in the assay medium using a crude extract of cassava (Manihot esculenta) leaf as source of invertase and amylase as well as in germinating cowpea (Vigna unguiculata L. Walp.) seedlings. There was a decreasing cowpea seedling invertase and amylase activities with increasing crude oil concentration in the soil. After three days of germination, the invertase and amylase activities in seedlings from 5.9% crude oil-treated soil decrease to 42 and 15%, respectively, compared to the control. A similar profile was obtained with invertase and amylase activities when crude oil was added directly to cassava extract enzyme assay. Addition of 10% crude oil in the assay inhibits invertase and amylase activities by 5.1 and 29.6%, respectively.