The article examines the pricing differences between two S&P 500 ETFs (ticker symbols SPY and IVV) and the underlying stock index. The author finds that, on average, both ETFs trade at a premium relative to the S&P 500; however, the level of the daily premium (and, on occasion, discount) varies between the two securities, which creates the opportunity for arbitrage. Since the passage of Regulation NMS in mid-2005, the pricing differences, as expected, have declined, implying that any current/future arbitrage opportunity will be confined to periods of high market volatility, such as 2008. Beyond issues related to arbitrage, the author finds that the relative pricing of the ETFs also provides a valuable signal of future (particularly next day) market activity. Thus, he suggests that active traders and longer-term investors may both benefit from recognition of relative ETF prices. Note: Link is to the article in a subscription database available to users affiliated with Butler University. Appropriate login information will be required for access. Users not affiliated with Butler University should contact their local librarian for assistance in locating a copy of this article.