Two Canadian universities conducted an experiment in which preservice teachers used computer-mediated communication to describe and discuss critical incidents occurring during their practicum. Participants in the practicum shared their teaching experiences with peers, cooperating teachers, and university supervisors. Over 3 years, 6 different discussion groups occurred, with 37 student teachers, 6 supervisors, and 6 cooperating teachers participating. More than 900 message exchanges took place during their 2-6 week practicums. Because participants had home and school access to the Internet and used a discussion group setup on the university server, the messages were available to all. Preservice teachers shared all the difficulties they experienced and reacted to critical incidents reported by others. Researchers analyzed critical incidents reported via the computer-mediated communication to identify the main types of problems and concerns. Data classification and analysis was based on a framework with four commonplaces of learning (teacher, student, subject, and context) which considered the main relationship between those four elements. Results indicated that the most problematic situations involved student misbehavior or student-teacher relations. Preservice teachers seldom expressed concerns for themselves or for students with learning disabilities. Supervisors used various supervision strategies and often used more than one in the same intervention. Their interventions did not reflect a unique pattern of supervision. Over time, their interventions became more elaborated. (Contains 12 references.) (SM)