‘Largo’ is an amphiploid betweenTrilicum turgidumL. andT. tauschii(Coss.) Schmal. This germ plasm line is highly resistant to greenbug,Schizaphis graminum(Rondani), biotypes C and E, which are currently the predominant biotypes of the Great Plains. Another biotype, designated as B, was prevalent on the Great Plains in the late 1950's and early 1960's, but is now difficult to find in the field. Since the possibility exists that B may again become predominant, this research was conducted to evaluate the reaction of ‘Largo’ to this biotype in comparison with a susceptible entry, ‘TAM W-101’, and OK 78047, which possesses a biotype C resistance gene derived from ‘Amigo’. Tests showed that the antibiosis and antixenosis of ‘Largo’ were considerably reduced compared with the ‘Amigo’ line, and it was also less tolerant of greenbug attack. Thus, lines with the ‘Largo’ gene would perform poorly in regions infested with biotype B.