MSMA (Monosodium methanearsonate) applied postemergence at 4.5, 6.7, or 9.0 kg/ha controlled wild oat (Avena fatua L.) in new plantings of Kentucky bluegrass (Poa pratensis L. ‘Pennstar’) and in perennial ryegrass (Lolium perenne L. ‘Reveille,’ ‘Pennfine’). MSMA at 4.5 kg/ha was almost as effective as the higher rates in controlling wild oat. MSMA at 2.2 kg/ha was less effective in some experiments. Visual injury to the perennial grasses was negligible. Satisfactory grass stands developed at all MSMA rates. Perennial ryegrass seed yields were higher than the untreated check in most experiments when MSMA was applied at 4.5, 6.7, or 9.0 kg/ha before the grass reached the boot stage. When MSMA was applied after the ryegrass reached the boot stage, some of the treatments reduced ‘Reveille’ perennial ryegrass seed production.