2,4-Dihydrox-7-methoxy1,4-benzoxazin-3-one (DIMBOA), a hydroxamic acid is considered to be one of the components responsible for the resistance to pest insects in cereal plants. The relationship between concentration of DIMBOA and aphid infestation on 21 corn lines was investigated in 1990 and 1991. DIMBOA was detected in leaves od all corn lines tested, contained larger amounts in young plants and gradually decreased with growth. This property was the same as observed in wheat. A more than ten times difference in DIMBOA concentration was observed in corn lines. However, there was no positive correlation between DIMBOA concentration and aphid density. Most of the lines used had resistance to aphids. Resistance may be built together with other components such as (E)-aconitic acid.