In this paper, we examined the within-person relationship between self-efficacy and performance in an Internet-based stock investment simulation in which participants engaged in a series of stock trading activities trying to achieve performance goals in response to dynamic task environments (performance feedback and stock market movements). Contrary to the results of several previous studies, we found that self-efficacy was positively related to effort and performance, and goal level partially mediated the efficacy-performance relationship. We also found that participants' affective reactions to performance feedback, measured as positive affect and negative affect, uniquely contributed to their motivation and performance either directly or by indirectly influencing their self-efficacy. (Contains 3 figures and 3 tables.)