How do instructors motivate students to participate in computer-mediated discussion? If they do participate, how can the quality of their interactions be assessed? This study speaks to these questions by examining online participation and discourse in a science course for preservice teachers. The instructor of an introductory entomology course for preservice teachers implemented online discussions by way of a listserv that was designed to provide students with greater access to important information outside of class. Data was collected from focus groups, written questionnaires, interviews with the instructor, and 182 public listserv messages. Initial student participation was encouraged by the instructor, but participation was modest. The posting of the first mandatory assignment halfway through the course, however, corresponded to a burst period of student activity, yielding a four fold increase in the number of messages authored by students. There was also a seven fold increase in the proportion of discussions that involved at least two student participants and a 50% increase in the proportion of outside references cited within the body of students' messages. This latter finding reflected improvement in the quality of online discourse among students. This evidence suggests that instructors who are interested in listserv participation should make some of their listserv activities mandatory.