NEW YORK ( TheStreet) -- Bank of America ( BAC - Get Report) saw a 21% rise in short interest during the first half of April as housing market weakness persists and investor concerns over the bank's exposure to mortgage-related troubles continues to grow.

Bank of America short interest climbed to nearly 96 million shares from just under 79 million at the end of March. The 21% rise was the most in percentage terms by any New York Stock Exchange-listed financial company during the first half of April, according to data compiled by Thomson Reuters.

Bank of America shares had lost about 7.5% over the past month as of late Friday morning trading. While that is far worse than JPMorgan Chase ( JPM - Get Report)'s drop of 0.11% over the same period, and a gain of 2.47% for Citigroup shares, it is slightly better than Wells Fargo ( WFC - Get Report), which has seen its shares lose 7.91% over that same stretch.

Both banks are heavily exposed to the U.S. housing market, though Bank of America's troubles are far greater, according to a report from Oppenheimer analyst Chris Kotowski.

Bank of America "services roughly as many delinquent mortgages as JPM and WFC combined and accounts for about two thirds of the industry's private label put back exposure," Kotowski wrote in an April 18 report.

Bank of America's $13.6 billion in put-back claims compares with less than $9 billion at JPMorgan Chase, Wells Fargo, Citigroup, Capital One Financial ( COF - Get Report), First Horizon National Corp. ( FHN - Get Report)and SunTrust Banks ( STI), according to a recent report from Nomura.

Bank of America's first quarter earnings report on April 15 appeared to be particularly troubling to analysts because its estimated "worst-case" exposure to disputes over mortgages underwritten in the last years of the housing bubble effectively grew, since it held steady at $7 billion to $10 billion despite a more than $1 billion settlement with monocline insurer Assured Guarantee ( AGO - Get Report).

Despite all the bad news, some analysts continue to recommend Bank of America shares. Jeff Harte argues that with the shares trading at less than six times his estimate of "normalized" earnings per share, investor fears are already priced in to the stock.

Short interest reflects the number of shares being lent out to investors betting against a rise in the stock of a particular company. Short sellers borrow shares in the hope the stock price will fall, so they can buy the shares at a lower price to repay the lender while pocketing the difference.

The NYSE releases short interest data twice a month with a roughly two week lag time for the 100 stocks with the greatest short interest outstanding.

-- Written by Dan Freed in New York.

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