The greenbug, Schizaphis graminum, is an important pest of grain and forage sorghum, frequently causing severe damage to sorghum stands and economic losses to producers. While greenbug-resistant sorghum hybrids are effective for managing greenbugs, the emergence of greenbug biotypes has allowed this insect to overcome many of the resistant cultivars. Previous studies have investigated the role of oxidative enzymes (catalase, polyphenol oxidase and peroxidase) in the defense response of sorghum resistant to greenbug feeding. No changes in the activity levels of catalase and polyphenol oxidase levels in control and greenbug infested sorghum plants were documented. However, gels stained for peroxidase activity identified a differentially expressed band in the isozyme profile of resistant sorghum. These results suggest that increases in peroxidases may accompany insect feeding in resistant genotypes and could serve as important markers for selecting resistant germplasm. This differentially expressed peroxidase was purified and sequenced to identify the specific peroxidase present. Our research used real-time PCR to quantify changes in peroxidase gene expression in greenbug-resistant (Cargill 607E) and –susceptible (Garst 5715) sorghum cultivars over time. The results of this research document similar trends in gene activity in 607E and 5715 at four and eight days after greenbug introduction. However, at twelve days after greenbug introduction there was a significant increase in peroxidase gene expression in the greenbug-resistant sorghum. These results strongly suggest that this peroxidase may play an important role in the defense response of sorghum to greenbug feeding and may serve as an effective marker for identifying sorghums with resistance to greenbugs.