This paper describes the use of facial electromyography (EMG) as a measure of positive and negative emotional valence during interactive experience. Thirteen boys played a car racing video game on an Xbox platform while facial EMG data were collected. Through video review positive and negative events during play were identified. The zygomaticus muscle EMG, which controls smiling, was found to be significantly greater during positive events as compared to negative. The corrugator muscle EMG, which controls frowning, was found to be significantly greater during negative events. The results of this study demonstrate that positive valence can be measured during interactive experiences with physiologic measures. This study also found that the corrugator EMG can still measure negative valence during high intensity interactive play in spite of the confounding factor of mental effort. These methods appear useful for associating the player's emotion with game events, and could be applied to HCI in general.