A field investigation was carried out to find out etiology for high morbidity and moderate mortality (7.50 %) in a desi chicken farm with strength of 3000 birds at the age of 12 weeks, which were reared in the deep litter system at Perambalur district, Tamil Nadu, India. The clinical signs noticed were ruffled feathers, weakness, anorexia and blood stained whitish to brownish diarrhoea and sudden mortality of healthy birds at the age of 12 weeks. Autopsy revealed that distended small intestine with red and white focal lesion on the serosal surface, fi lled with blood, mucus and tissue debris. The microscopic examination confirmed the presence of Eimeria necatrix. The fl ock was treated with amprolium hydrochloride with vitamin K and the mortality reduced and the fl ock recovered completely on 5th day of treatment. Though, the desi chicken is deemed to be resistant to coccidiosis, the breed upgradation, wet deep litter condition and poor ventilation might play the important role in the coccidiosis outbreak among desi chickens.