For 40 cases of chronic otitis media with chronic mastoiditis, for whom mastoidectomy was done at department of otolaryngology, Seoul national university hospital from october 1965 to march 1966, bacteriological studies for the floras in the mastoid cavities were performed. Following results were obtained ; 1) Pseudomonas had the highest relative abundance (30%) and the number of staphylococcus was relatively high (27.5%), while no growth was found in 5 cases in the pure culture. 2) In the sensitivity tests for antibiotics, staphylococcus was relatively sensitive to many types of antibiotics and the others, especially pseudomonas and proteus, were almost resistant to many types of antibiotics.