The corn leaf aphid, Rhopalosiphum maidis (Fitch) (Homoptera: Aphididae), is an occasional pest of maize, Zea mays L. Corn leaf aphid damage in maize has been characterized in whorl-stage plants, but seedling damage has not been investigated. Resistance mechanisms in maize to corn leaf aphids are not well understood. The objectives of these studies were to determine the role of DIMBOA (2,4-dihydroxy-7-methoxyl-1, 4-benzoxazin-3-one) in maize resistance to corn leaf aphids, to document corn leaf aphid feeding damage on seedling maize, and to characterize feeding behavior differences on seedling and whorl-stage maize;Inbred 41:2504B (high DIMBOA) was colonized by corn leaf aphids immediately upon seedling emergence. Inbreds B73 and B37 (low DIMBOA) were colonized in the three- and six-leaf stages, respectively. Inbreds C103 and Mo17 (low DIMBOA) were colonized in the six- and seven-leaf stages, respectively. Regression and partial correlation analysis showed no relationship between corn leaf aphid colonization and DIMBOA. These data indicate that DIMBOA is not a primary resistance factor in maize to corn leaf aphids;Corn leaf aphid feeding on coleoptile and two-leaf stage maize plants reduced plant height and delayed pollen shedding and silking. Grain fill was reduced when aphids fed on coleoptile, two-leaf, and four-leaf stage maize. Grain fill reductions occurred on inbreds B37, C103, and 41:2504B, but not on B73 and Mo17. Corn leaf aphid damage on seedling maize resulted in yield reductions;Examination of late-whorl stage and seedling maize indicated that 57% of the corn leaf aphid stylet tracks penetrated late-whorl stage plants through stomata. Eighty-six percent of stylet penetrations in seedling maize occurred intercellularly between epidermal cells and 8% through the stomata. Fifty-seven percent and 79% of the stylet tracks terminated in phloem in whorl-stage and seedling maize, respectively. Aphid damaged stomata may be important in explaining yield reductions in heavily infested water stressed maize;These findings indicate that corn leaf aphids were capable of feeding on and damaging seedling maize, and that resistance mechanisms in maize to the corn leaf aphid do not involve the secondary plant chemical DIMBOA.