A pen of approximately 50 conventionally raised Holstein heifers between 230 and 270 days old presented for sudden morbidity and mortality of unknown origin. A diagnosis of coccidiosis was made based on history, physical examination, and fecal results. Disease occurred despite prophylactic treatment with the coccidiostat Rumensin (monensin) at a target dose of 200 mg/head/day. Clinical signs resolved and fecal oocyst counts decreased after sick animals were treated with Corid (amprolium) and the Rumensin (monensin) dose was increased to 300 mg/head/day. Affected animals continued to show signs of immune compromise following the outbreak but no other animals in the barn were affected.