The credibility of an instructional simulation is the most important issue in distance education, where it may replace hands-on activities. This credibility is based in large part upon verisimilitude, a perception strongly influenced by the simulation's realism. This paper presents a case study encompassing the design process of a simulation-based virtual laboratory, which was guided by a realism principle, and an investigation of its credibility among potential users. We found that many characteristics of the environment associated with the design principle favoured its credibility, but that others had widely varying, even opposite, effects among users. A user's prior experience was shown to play a crucial but intricate role in verisimilitude and credibility judgements.