A histopathological change of osseous tissues of the temporal bone in experimentally produced Pseudomonas otitis media was studied in 30 rats. Otitis media was developed by innoculating Pseudomonas aeruginosa suspension(10°/ml) into the bulla through the tympanic membrane. In the period of 3 days to 12 weeks after innoculation, the histopathological changes of the bony wall of the anterior bulla, the ossicle and the bony cochlear promontory were studied with a light microscope. In the anterior bullar bones from the 3 days animals, new bone formation with osteoblastic reaction was noted. One week animals showed trabecular bone formation and focal fibrosis. In the 2 weeks rats, the proliferation of osteoblasts in the inner surface of the affected bones resulted in moderately increased new bone formation. In the 4 weeks rats, distinct cement lines due to markedly increased new bone were observed. There were also found proliferation of epithelial cells, necrotic tissues and fatty degeneration in the subepithelial space. The 6 weeks rats showed mild osteoclastic activity in both inner and outer surface. The 8 weeks rats showed typical and distinct cement lines, moderate degree of fibrosis on both surface of the bulla, and necrotic tissues in the subepithelial space. In the 12 weeks rats, the anterior bullar bony walls were thin and reduced to nearly normal status. No further activities of osteoblasts and osteoclasts were found. But the fatty degeneration and fibrosis in the subepithelial space continued. In the ossicles, the 3 days rats showed pyogenic inflammatory findings such as pus cells and granulation tissues surrounding the ossicles. The 2 weeks rats showed bone resorption mainly due to inflammatory reaction, not to osteoclastic activity. The 4 weeks rats showed that the ossicles were surrounded by severely infected granulation tissues. In the 8 weeks rats, the ossicles were severely destructed and replaced by granulation tissues. In the cochlear promontories, bone resorption due to mild osteoclastic activity began to be observed in the 2 weeks rats. But, an inflammatory infiltration caused the bone resorption and it continued through the 8 weeks rats.