After a temporary drop in the last six weeks of 2013, short interest is back on the rise.
The NYSE reported on Monday that short interest rose to 13.8 billion shares in the first half of January from 13.41 billion on December 30, a 2.98 percent increase – the biggest such bi-weekly increase since mid-June of last year.
Likewise, Nasdaq also showed a modest increase of 0.5 percent in short interest during the same period. As of mid-January, short interest rose to about 7.62 billion shares, compared with 7.58 billion shares as of December 31.
(Read more: Betting against Apple? Not so fast: Ex-CEO Sculley)
Short interest measures the total number of shares of a security that has been sold short, expressed as a percent of total tradable shares.
Investors track short-interest levels to gain a sense of where a stock might be headed, along with some insight into whether any positive news might force short traders to cover their positions, pushing stocks higher.
Here is a look at the most heavily shorted stocks:
—By CNBC's Giovanny Moreano. Follow him on Twitter: