This study focused on deviant behaviour engaged in by library users in the course of their interaction with the library systems of Ajayi Crowther University and Emmanuel Alayande College of Education, Oyo, Oyo State-Nigeria. The study investigated the kinds and causes of deviant behaviour as well as the measures put in place to drastically reduce or eliminate these behaviour by the patrons of the libraries under investigation. The sample consisted of 50 library staff and 200 library users. Questionnaire was used as instrument of data collection while percentage count and tables frequencies were used for the analysis and presentation of data. It was revealed from the study findings that theft, mutilation, impersonation, and defacement of information materials were the most commonly committed deviant behaviour; while factors like poverty, laziness, natural instinct and poor reading culture were responsible for the deviant behaviour. The study therefore recommends that libraries, especially academic libraries put in place effective and efficient library security network, do frequent library orientation and information literacy for new and old users from time to time. There should also be provision of free library guides as some short and long-term measures that could help reduce the rate of deviant behaviour among library users.