Flow, "the optimal experience," is a state of pleasant deep-focus on the task at hand that is both challenging and rewarding. Naturally, flow is a hot topic in videogame research. The purpose of this study is to determine what impact a generative music system embedded in a videogame would have on the subjective experience of flow. In this study, participants played two versions of a Tetris-like game. One version had a generative music system for the music soundtrack, and the other had a traditional interactive soundtrack. Participants that played the version with the generative music system first reported flow more than those that played the traditional soundtrack version first. Participants were not made aware of the manipulation, nor were they capable of correctly identifying any differences between the two versions when asked. Another purpose of this study was to begin to bring together disparate bodies of literature: musicology, experimental psychology and music theory. These fields have much to offer each other, but are not often enough connected.