Recent changes in the media environment have generated serious debates about whether these changes will foster or hamper the proper functioning of democracy. Along this line, the tendency toward audience fragmentation has arisen as one of the main concerns that might undermine a healthy democracy. People are not likely to discuss common social problems and understand each other in fragmented society, much less agree on methods to solve them. This dissertation investigates whether the environmental changes result in the audience fragmentation. This study first clarifies conceptual and operational definition of fragmentation. It is conceptualized as division of the general public into small groups not communicating with each other, and operationalized as existence of a common agenda by means of incidental news exposure facilitated by structural factors of online communication, an exemplar of the new information environment. Data from several different methods are employed to investigate effects of new media on fragmentation: a content analysis, a survey, and a laboratory experiment. The results provide evidence supporting that the public still can share experiences by learning a common agenda from the media on the Internet, the medium considered an icon of the new media environment. Findings of the content analysis found significant positive correlations between news agenda of different media outlets, indicating that there is a common agenda in the media. Environmental factors of the media environment rather than individual differences in political predispositions have strong influence on people’s incidental news exposure, a key route to acquire a common agenda. A series of analyses based on the survey found that overall frequency of Internet use significantly predicted individuals’ reports of incidental news exposure online, whereas there was no significant relationship between political predispositions and incidental exposure. It also appears that certain online activities such as getting entertainment/sports information significantly predicted the incidental news exposure. The incidental news exposure was found to have actual effects on people’s learning of a common agenda and recognition/recall of information carried by stimulus messages in the experiment. The implications of the findings are discussed in terms of communication research and media/democracy. / text