This summer's wave of bank mergers has drawn not only investors expecting stronger bank stock prices, but also those betting some banks' shares will weaken.

Short selling, typically a bet by investors that share prices will fall, increased 223% in the last month with PNC Bank Corp., for example, after the bank agreed to a $3 billion acquisition of Midlantic Corp.

Subscribe Now

Access to authoritative analysis and perspective and our data-driven report series.

14-Day Free Trial

No credit card required. Complete access to articles, breaking news and industry data.