The greenbug, Schizaphis graminum (Rondani), is a major pest of wheat in North America, reducing U.S. wheat production by $60 to $100 million each year. Experiments were conducted to determine the categories of resistance to greenbug biotype I controlled by two different resistance genes in wheat germplasm lines ‘KS89WGRC4’, containing the Gbx gene, and ‘Sando's 4040’, containing the Gby gene. Antixenosis (non-preference), antibiosis (lowered greenbug intrinsic rate of increase) and tolerance (reduced plant tissue and chlorophyll loss) assays were conducted using plants of Sando's 4040, KS89WGRC4, ‘Jagger’ (susceptible control), ‘Largo’ (antibiosis control), and ‘TA1675’ (tolerance control). Neither Sando's 4040, KS89WGRC4 nor the controls exhibited antixenosis to greenbug biotype I. There was an antibiotic effect on the greenbugs confined to Sando's 4040 (r_m = 0.122), that was no different than the r_m of aphids on the resistant control, Largo (r_m = 0.144). Antibiosis was not present in KS89WGRC4. Both Sando's 4040 and KS89WGRC4 exhibited tolerance to greenbug biotype I feeding damage, based on measurements of proportional dry plant tissue weight change and leaf chlorophyll loss. Sando's 4040 and KS89WGRC4 provide useful new sources of resistance to greenbugs for wheat breeding programs.