A case of pneumonia caused by C. pneumoniae, strain TWAR is described in this paper. A 65 year-old male with a persistent dry cough was admitted to our division for left lower lobe infiltrates of the chest X-ray. The serum antibody titers against mycoplasma and some viruses were not elevated, but the serum antibody titers against TWAR reached the maximum level (IgG X 1024, IgA X 256) using microplate immunofluorescence antibody technique (MFA). Isolation of TWAR was tried by BAL and nasophalingial swabs, but were not successful. TBLB from Lt. S10 revealed TWAR inclusion bodies within alveolar epithelial cells using TWAR specific monoclonal antibody (Washington Research Foundation).