Background/Aims:Moraxella catarrhalis is an important and common pathogen of humans.The organism colonizes the upper respiratory tract of children and adults in the absence of clinical infection.Approximately 1 to 5％ of healthy adults are colonized with Moraxella catarrhalis.Adults with chronic lung diseases appear to have a higher rate of respiratory tract colonization than healthy adults.It is a common cause of respiratory infections,particularly otitis media in children and lower respiratory tract infections in the elderly.