Interactivity is defined by Henri (1992) as a three-step process involving communication of information, a response to this information, and a reply to that first response. It is a key dimension of computer-mediated communication, particularly in the one-on-one communication involved in an electronic mentoring program. This report analyzes the interactivity between pairs of corporate research scientists (mentors) and university biology students (proteges) during two consecutive implementations of an electronic mentoring program. The frequency and structure of the interactions within each pair were examined to provide context: 542 messages were posted among the 20 mentors and 20 proteges. These messages were formed into 5-10 threads per pair, with 3-4 messages per thread, indicating a high level of interactivity (there were more responses posted than independent messages). Mentor-protege pairs rated as effective by both mentors and proteges posted more messages overall, had well-structured threads, had protege and mentor postings that were similar in topic coverage and message length, and had little overt "management" behavior by mentors. However, there appears to be no clear recipe for successful interaction. Not only are there a variety of factors at play in developing an online relationship in this context, but mentor-protege pairs can falter at various stages in the process and in various ways.