A survey is given of the literature concerning yeast mastitis in cows. Routine mycological examinations of 980 milk samples from clinically normal quarters indicated that yeasts do not belong to the normal microflora of the udder. 5 cases of mycotic infections were found in 480 samples from abnormal quarters. A description is given of a total of 7 cases of udder infections from which the following agents were isolated: Saccharomyces marxianus, Saccharomyces fragilis, Candida krusei, Candida parapsilosis var. intermedia and a Trichosporon sp. Experimental infection of cows with 2 of the isolated strains produced distinct local and general symptoms. In vitro sensitivity tests revealed no effect of penicillin, streptomycin, chloramphenicol, aureomycin and terramycin on any of the mastitis strains. The 2 latter antibiotics showed a marked growth-stimulating effect. Polymyxin B, nystatin and trichamycin proved to have an effect on all strains. A list of yeast species reported from mastitis cases demonstrates the rich variety of agents implied, the habitats and the pathogenic properties of which are discussed. Attention is called to the role of the extensive use of antibiotics as a factor explaining the increasing number of mycotic mastitis.