Stocks investors are shorting the most

Traders on the floor of the New York Stock Exchange on Monday
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Traders on the floor of the New York Stock Exchange on Monday

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.

Cliffs Natural Resources, U.S. Steel, Lennar and D.R. Horton, for example, have some of the highest ratios in the S&P 500 index, with more than 18 percent of their float sold short.

Here is a look at the most heavily shorted stocks:

Most shorted stocks in the S&P 500

YTD % Chg.
Short Interest
CLF Cliffs Natural Resources Inc. $20.92 -20.2 34
GME GameStop Corp. Class A $36.95 -25 25.1
X United States Steel Corporation $25.70 -12.9 22.7
ADT The ADT Corporation $38.14 -5.8 20.4
FTR Frontier Communications Corporation Class B $4.93 5.9 19.8
JOY Joy Global Inc. $53.73 -8.1 19.7
LEN Lennar Corporation Class A $38.99 -1.4 19.4
CVC Cablevision Systems Corporation Class A $16.43 -8.4 18.7
DHI D.R. Horton, Inc. $22.95 2.8 18
TRIP TripAdvisor, Inc. $77.96 -5.9 17.7
VRSN VeriSign, Inc. $59.92 0.2 16.8
FSLR First Solar, Inc. $50.10 -8.3 16.5
DGX Quest Diagnostics Incorporated $53.35 -0.4 15.7
WIN Windstream Holdings, Inc. $7.52 -5.8 14.6
PBI Pitney Bowes Inc. $21.74 -6.7 13.9
GRMN Garmin Ltd. $45.68 -1.1 13.6
TSN Tyson Foods, Inc. Class A $34.92 4.4 13.5
HAS Hasbro, Inc. $51.77 -5.9 12.1
EA Electronic Arts Inc. $24.57 7.1 11.7
EXPE Expedia, Inc. $66.80 -4.1 11.4
All data as of 11 am EST. Source: CNBC Analytics

—By CNBC's Giovanny Moreano. Follow him on Twitter: @giovannymoreano