The study was carried out in a herd of 280 dairy cows in the North-Eastern part of Poland in the summer of 2011. During the period of 5-6 months before the study, mastitis cases resistant to routine antibiotic therapy were observed in this herd. Bacteriological examination of 280 milk samples collected from 70 cows with clinical or subclinical forms of mastitis was performed. Diagnosis of mastitis was made on the basis of anamnesis, clinical examination of the udder, macroscopic evaluation of secretion, California mastitis test (CMT), and results of bacteriological examination of milk. Protothecal mastitis was detected in 34 cows (12.6% of all cows in the herd). Algae belonging to Prototheca zopfii were isolated from 27 milk samples in pure cultures; in the remaining seven samples, mixed infections were identified (P. zopfii and Staphylococcus sp.). The acute form accompanied by elevated body temperature (40 C), pain and hot oedema of the udder, loss of appetite, and reluctance to move were observed in two cows immediately after delivery. The similar symptoms were also noted in three cows with mixed infections. The chronic form of protothecal mastitis was characterised by pasty oedema in the udder of slight painfulness and hard tissue consistency, as well as markedly reduced milk secretion. The macro- and microscopic changes in the mammary tissue was indicative of chronic, fusing microgranulomatous interstitial mastitis protothecosa.