The purpose is to present library OPAC as a communication genre in its mutability. The paper is based on the idea of OPAC development as a transition to subsequent OPAC generations. Every generation, in the light of genre theory, can be treated as a subgenre with its own communication purpose. As such, it is subject to transformations caused by information technology development. OPAC development is described as an electronic genre transition process, which allows for distinguishing eight OPAC subgenre generations. They were distinguished based on socio-historical development of the genre system and were described according to Shepherd and Watters1genre development model. These subgenres are then subjected to genres analysis revealing their basic characteristics (purpose, form and functionality). The paper is divided into two parts. Part 1 describes text genre theory and OPAC generations historical development as communication genre. Part 2 is dedicated to OPAC genre analysis.