Professional associations could use virtual communities to reinvent their relationships with their members. One promising building block would be to revitalise Special Interest Groups as loose knit Internet based communities. These would be what has been called "networks of practice" or "communities of interest". Although recognised phenomena, these terms are little more than labels, however. Far more has been written about the rich, close knit relations of communities of practice. This paper argues that as a model the community of practice concept is useful in defining dimensions by which such looser knit groups can be studied. To this end, community of practice theory is reviewed. The paper then presents a case study of an informal community among web developers. It uses genre analysis to identify a dominant discourse which constitutes the list?s local culture or repertoire. The genres in use are very efficient means of exchanging information, offering broad learning opportunities. They also construct a member identity. They exclude social and political issues, constructing "the problem" of the web as technical. The "network" has a sense of community, yet it does not have the level of engagement of a community of practice because of the lack of task interdependence, the limits of the technology in use and the size of membership. This has some advantages such as giving it a longer life, requiring less commitment from members and coordinators and making it scale to numbers in a professional association. Professional associations could build up from such networks of practice, just as corporations have sought to cultivate spontaneous informal groups, to manage knowledge.