Aberdeen Angus (AA) and crossbred AA x Hereford 8-9 mo old heifers (560 animals) showed a typical perennial ryegrass (PRG) staggers syndrome after grazing 26 d in a 25-hectare paddock of pure Lolium perenne. Effects were observed in about 50% of the herd. When forced to move, the animals showed incoordination, weakening of the forelimbs, falling on the chest and neck, and moving in this position during the first steps before recovering a normal gait. In severely affected animals all of the 4 limbs had extreme rigidity. All the affected animals had generalized tremors. Twelve days after removal from this pasture the animals had no symptoms. The symptoms described, the pasture conditions (mostly PRG stubble and short PRG green plants in slow growth), a positive mouse bioassay of extracts obtained from the basal part of the PRG plants, and the microscopic finding of Acremonium lolii hyphae in the leaf sheaths of the same plant material produced diagnostic evidence to confirm this as a PRG staggers case, the first reported in Argentina.