Though video games are commonly considered to hold great potential as learning environments, their effectiveness as a teaching tool has yet to be determined. One reason for this is that researchers often run into the problem of multicollinearity between prior knowledge, in-game performance, and posttest scores, thereby making the determination of the amount of learning attributable to the game difficult. This study uses tests for mediation effects to determine the true relationship between in-game performance and posttest performance, determining that in this case in-game performance is a perfect mediator of prior knowledge on posttest score. (Contains 1 table, 3 figures and 1 footnote.) [This paper was supported under the Educational Research and Development Centers Program.]