Pathologic changes of the round window membrane was studied by electronmicrorscopic examination after innoculation of 0.1 ml of Pseudomonas aeruginosa(P-13) at a concentration of 10⁹ organism/ml into the bulla of the guinea pig through the tympanic membrane. The results were as follows : 1) Normal round window membrane of guinea pig had three layers, outer epithelial layer, middle fibrous layer, and inner mesothelial layer. The epithelial cell had microvilli on its free surface, few mitochondrias, and rough endoplasmic reticulum in the cytoplasm. Cell junctions were composed of zounla occludens, zonula adherens, and abundant desmosomes. Continuous basement membrane was found beneath the epithelial cells. In middle layer, fibroblasts, collagen fibers, elastic elastic fibers, and capillaries were found. In mesothelial layer, basement membrane and microvilli were absent. 2) In three days animal, the round window membrane was markedly thickened, and epithlial cell showed mild hyperplasia. Golgi body and rough endoplasmic reticulum were slightly swollen. Cell junctions were intact. Microvilli showed sloughing, fusion and branching. In middle layer, severe edema, proliferation of fibroblasts and infiltration of polymorphonuclear leukocytes were found. 3) In seven days animal, hyperlasia of the epithelial cells, and swelling of Golgi body and rough endoplasmic reticum were more prominent than three days animal. There were numerous free ribosomes and vacuoles in epithelial cells and mesothelial cells. Cell juctions were intact. Microvilli showed sloughing, fusion and branching. Middle layer was markedly thickened due to infiltration of polymorphonuclear leukocytes and proliferation of fibroblasts, collagen fibers, and elastic fibers. 4) The round window membrane is one of the most important conduit for labyrinthitis and sensorineural hearing loss secondary to otitis media. Early management otitis media is essential to prevent these complications.