Prevotella are gram negative, obligate anaerobes and includes pigmented and non-pigmented species. They are typically regarded as commensals, especially of the gastrointestinal tract and oral cavity, although opportunistic endogenous infections are known to be caused by a small number of strains. We report a case of a 51-year-old man with right-sided chest pain. Chest X-ray showed mild haziness in the right lung and he was diagnosed with right sided pleural effusion. Prevotella oris was isolated after pleural pus culture.