At the Quality Milk Production Service of the College of Veterinary Medicine Diagnostic Laboratory at Cornell University, several dairy farms in upstate New York have been identified with a large percentage of clinical and subclinical cases of Prototheca mastitis. In this study, one such farm is evaluated through bacteriologic herd serveys, environmental sampling, and review of management techniques. The information collected is designed to assist the farmer in identifying sources of Prototheca infections and methods of controlling further outbreaks. A total of 33 (24%) Holstein dairy cows cultured positive for Prototheca in composite herd survey milk samples. Inflation swabs performed following milking of these 33 cows and random non-positive cows showed 83% were positive for Prototheca. Fecal culture of these same 33 cows as well as random cows not previously positive for protothecal mammary gland infections, showed that 35% had Prototheca actively passing through their gastrointestinal system. This is a major source of environmental contamination by Prototheca. Of the 23 environmental samples collected, 12% cultured positive for Prototheca. These results may, in fact, be higher, as fungal contamination of cultures may have prevented identification of some Prototheca colonies.