(Adds details on Gross' investment strategy, analyst quote, background)
NEW YORK, June 12 (Reuters) - Billionaire Bill Gross' bond fund posted $300 million in withdrawals in the month of May, when it turned in its worst single-day performance since launching in 2014, data from research service Morningstar Inc showed on Tuesday.
Gross runs the Janus Henderson Global Unconstrained Bond Fund, which had assets of $2.1 billion at the end of April, and was hit by sharp declines last month when bond market volatility surged on Italian political concerns.
Low-cost shares of the fund are down 4.25 percent this year through Monday, according to Thomson Reuters' Lipper research unit, near the bottom of its peer group.
Investors have fretted over Italy's difficulty installing a government, especially concerned that leaders of the euro zone country would try to leave the single-currency bloc or massively increase fiscal spending. Investors have fled Italian debt and flocked to safe-haven German bonds.
Yields on benchmark 2-year Italian bonds spiked from below zero percent on May 15 to as high as 2.7 percent by the end of the month, while the same type of German bonds moved from a high of negative 0.52 percent to as low as negative 0.85 percent during the month.
When investors clamor for more bonds, they bid up prices and yields fall. The opposite is true when investors rush to sell, sending yields higher.
Earlier this month, Gross told Bloomberg News that he has been betting that German government bond prices would fall while positioning for U.S. Treasuries to rise, adding that he expects to recover from what he described as a "bad trade."
"Investors in bond funds do not like such volatility particularly in a bond fund that is expected to generate stable income streams," said Todd Rosenbluth, director of mutual fund research at New York-based CFRA Research.
A Janus Henderson spokesman did not immediately respond to a request for comment.
Gross, once considered Wall Street's "Bond King," co-founded Newport Beach, California-based Pacific Investment Management Co in 1971 but left abruptly in 2014 after a management rift and moved over to Janus. Gross is worth $2.5 billion, according to Forbes magazine. (Reporting by Jennifer Ablan and Trevor Hunnicutt Editing by Jeffrey Benkoe)