Naked short selling occurs when a short seller fails to deliver shares on the settlement day. The business press and many corporate managers characterize it as abusive price manipulation, alleging that selling nonexistent shares causes a price decline regardless of fundamentals. To curtail the practice, the SEC issued regulations, first to restrain and later to prohibit all naked short selling (i.e., Reg. SHO in 2004 and its amendment in 2009). Contrary to allegations, I find that the naked short selling component of total short interest is significantly associated with accounting and market fundamentals, indicating proper information usage. Further, naked short interest is highly significant in predicting one-quarter ahead abnormal stock returns, and it dominates covered short interest when both measures are included. I also calculate returns from a zero-investment trading strategy that buys (sells) shares with low (high) levels of both covered and naked short interest. I find abnormal returns are approximately 3.9 times larger than when using only covered short interest. Empirical evidence therefore indicates that recent actions by regulators to eliminate naked short selling are likely to impede arbitrage and thereby reduce market efficiency.