Lasalocid sodium was accidentally introduced into the feed of several broiler breeder chicken farms at levels between 115 and 150 ppm. On one farm, leg weakness and ataxia were observed in a few cockerels. A reduction in egg production and a sharp decrease in fertility and hatchability were observed in all the flocks receiving the contaminated feed. Many piping chicks were unable to hatch and there was an increase in the number of weak ataxic chicks at the hatchery. Histological examination of the muscle tissues of the affected cockerels, the piping chicks unable to hatch and the one-day-old chicks with leg weakness and ataxia revealed severe muscle damage. Increasing levels of lasalocid were detected in the yolk of eggs collected from the affected flocks.