Qualitative and quantitative examinations of the cultivable bacterial flora in biopsies from recurrent aphthous ulcerations (RAU), experimental oral ulcerations (EOU), and normal oral mucosa (NOM) were carried out under aerobic and continuous anaerobic conditions. An attempt was made to culture yeasts, mycoplasma, and virus from the biopsies of RAU, which were also tested for the presence of herpes virus antigen by the fluorescent antibody method. The predominant bacteria recovered in RAU were alpha-hemolytic streptococci, coagulase-negative staphylococci, and Neisseria. In EOU the main recoveries were alpha-hemolytic streptococci, Corynebacterium, Veillonella, Neisseria, and Haemophilus. In NOM alpha-hemolytic streptococci dominated the cultures. Yeasts were only cultured from one patient with RAU. No mycoplasmas or viruses were isolated, nor was herpes virus antigen demonstrated in any of the RAU specimens. The role of the microorganisms in the pathogenesis of RAU is discussed.