So much for a rebound at billionaire John Paulson’s hedge fund firm.

Paulson & Co.’s main merger fund, Paulson Partners, fell 6 percent in September, cutting its 2015 gain to 0.6 percent, according to a person with knowledge of the matter. The merger strategy is the firm’s largest.

September’s losses are eroding what had been a bright spot for the $19 billion firm as it tries to recover from its second- worst year ever in 2014. Paulson Partners had gained almost 9 percent this year through July, before starting to lose money in August as volatility struck global markets. Hedge funds on average lost 0.6 percent last month, cutting their 2015 gains to 0.7 percent, according to data compiled by Bloomberg.

Paulson’s Advantage hedge fund, which makes bets on companies undergoing events such as bankruptcies and spinoffs, has fared worse. It lost 8.5 percent last month, said the person, who asked not to be named because the information isn’t public. The Advantage fund was down 12 percent this year through September.

Armel Leslie, a spokesman for New York-based Paulson with Peppercomm, declined to comment on the performance.