Spring barley often is grown in rotation with winter wheat, and sometimes barley can overwinter in the subsequent winter wheat crop reducing grain yield and quality. Studies were established during 1996 and 1997 in winter wheat fields in southeastern Washington and near Moscow, ID, respectively, to evaluate control of ‘Steptoe’ volunteer barley with MON 37500, diclofop, and fenoxaprop/2,4-D/MCPA. Herbicides were applied to volunteer barley at two growth stages: two leaves to four tillers and more than four tillers with stems beginning to elongate. MON 37500 at 0.018, 0.026, and 0.035 kg ai/ha visibly controlled volunteer barley 83% or more at both application times. Diclofop at 1.12 kg ai/ha did not control volunteer barley, whereas fenoxaprop/2,4-D/MCPA at 0.66 kg ai/ha controlled volunteer barley 64 to 97% in 1996, but only 0 to 23% in 1997. In 1996 and 1997, volunteer barley density was reduced 80 to 99% in MON 37500-treated plots compared to the untreated control plots. Wheat grain grade was #1 for all MON 37500 treatments compared to grade #4 in 1996 and #3 in 1997 in the untreated plots. Grain price was reduced by dockage (barley kernels) for MON 37500-treated wheat $0 to $3.12/metric ton (MT), whereas price was reduced $23 to $26/MT for grain from untreated plots. In greenhouse studies, visible injury and height and biomass reduction varied among the 36 barley varieties treated with MON 37500.