The mastitis caused by P. zopfii alga is a disease of high-producing, machine-milked dairy cows. It occurs worldwide in tropical and temperate climatic areas, and mostly appears sporadically in a therapy-resistant form. However, in poorly managed dairy herds it may be endemic, causing serious economic losses as a result of decreased milk quality and quantity and culling of infected animals. The biological properties of this pathogenic alga, the laboratory methods available for its isolation and identification, the pathological and clinical features of this form of mastitis, and the principles of its control are reviewed in this paper.