Given the increase in protothecal mastitis worldwide, the lack of effective therapy for mammary infections, and severe losses to the dairy industry related to bovine mastitis-related Prototheca species, we investigated the in vitro algaecide activity of QA against Prototheca isolated from clinical bovine mastitis, identified at the species-level based on mass spectrometry and PCR sequencing of the cytb gene. The gene discussed is MT-CYB; the disease is mastitis.