NEW YORK ( TheStreet) -- Short interest in Berkshire Hathway ( BRK.B) more than doubled in the second half of June, just days before an analyst put a rare "sell" recommendation on the stock. Short interest on Berkshire Hathaway "B" shares rose to nearly 52 million at the end of June from 22 million at the middle of the month, according to New York Stock Exchange data released late Monday. Stifel Nicolaus analyst Meyer Shields downgraded Berkshire to "sell" from "hold" on July 8, arguing a weaker economy will hurt the shares of the conglomerate. Analysts generally put very few "sell" recommendations on stocks as a rule. Goldman Sachs analysts took the opposite view, initiating coverage of Berkshire with a "buy" on June 29. Berkshire shares have outpaced the S&P 500 by a wide margin in 2010, gaining more than 20% while the broader index has lost ground. The trend has reversed somewhat in July, with Berkshire essentially flat so far on the month while the S&P 500 has gained more than 6%. Berkshire "B" shares were the 26th most-shorted ticker symbol on the NYSE in the latter half of June. In contrast to Berkshire, which according to Reuters, saw a bigger percentage gain in short interest than any other NYSE-listed company, Citigroup ( C) saw the biggest drop in percentage terms. Citigroup short interest fell to 433 million shares at the end of June from 497 million. Citigroup remains the most heavily-shorted listing on the NYSE, a distinction it wins regularly, and which is consistent with the fact that it is also the most actively-traded stock on most days. The drop in Citigroup short interest is consistent with the fact that the bank's shares have been a strong performer over the past month, gaining more than 9% and outpacing competitors like Bank of America ( BAC), Wells Fargo ( WFC) and JPMorgan Chase ( JPM). -- Written by Dan Freed in New York.