As the S&P 500 continues to trade near record highs, short interest appears to be on the rise.
According to data from FactSet, in the first half of July average short interest for S&P 500 stocks increased more than 5 percent, with over 55 percent of those companies seeing a spike in short interest.
Short interest measures the total number of shares of a security that have 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.
Over the last year, Cliffs shares have fallen 55 percent, as profits sank and the company closed or cut production at U.S. mines and trimmed its dividend. U.S. Steel is also down sharply, though Pitney Bowes has bucked the trend with a double-digit gain.
Across sectors, investors are most bearish on telecoms—where more than 8 percent of shares have been sold short—materials stocks and consumer discretionary names.
But the short sellers have largely stayed away from utilities, financials and consumer staples; in all three cases short interest is less than 3 percent.
Here is a look at some of the S&P 500 companies that have the biggest short interest ratio.
—By CNBC's Giovanny Moreano and Pradip Sigdyal