This case report describes the first case of mastitis in Finland caused by the alga Prototheca zopfii in one cow in a small herd. Prototheca zopfii causes cases of subclinical or clinical mastitis. Milk production decreases precipitously, cell count increases and lesions appear in the udder tissue. It is refractory to any treatment. Prototheca zopfii has been isolated from a variety of environmental sources. Inadequate housing and poor milking hygiene contribute to susceptibility. Presumptive identification of Prototheca is based on characteristic cell morphology of the alga.