Peracute onset of disease was reported in a 42-wk-old broiler breeder flock that was presented by error with feed containing monensin at approximately seven times the approved level for broiler chickens. Morbidity and mortality were extremely high, and the affected chickens displayed feed refusal, decreased water consumption, and severe paralysis that ranged from abnormal gait to a complete inability to move. During the first 10 days postingestion of the suspect feed, mortality in hens reached 13.7% and 70.9% in the roosters. Hen day production decreased from 67% to 3% in the same period of time. A total of 638 g/ton of monensin was detected in suspect feed samples by one laboratory and 740 g/ton in a second laboratory. Twenty-one days after removal of the suspect feed, the mortality rate returned to normal levels in both hens and roosters, albeit feed consumption and egg production remained extremely low, which prompted the company involved to eliminate the flock.