Sluggish markets and the May 6 flash crash pinched even highflier Goldman Sachs Group Inc in the second quarter, according to the company's quarterly report filed on Monday.

The New York-based investment bank reported 10 days of trading losses in the period, including losses of more than $100 million on three days.

In the first quarter, Goldman hit trading perfection by reporting trading gains of at least $25 million every day.

Goldman's trading gains and losses have mirrored those of its Wall Street counterparts: Surging first-quarter trading results cooled in the spring and early summer.

The S&P 500 Index <.SPX> fell 12 percent in second quarter as the flash crash, the debate in the U.S. Congress over financial reform, and renewed investor fears about U.S. economic growth resulted in low returns and high volatility.

Analysts and investors said Goldman's second-quarter results were not surprising, given the quarter's trading headwinds.

It speaks to the difficulty of the markets we're in right now, said Walter Todd, co-chief investment officer at Greenwood Capital & Associates. It's a very tough market for anyone to figure out and try to make any money.

While the second quarter was still largely profitable for Goldman's trading operations, positive trading days seesawed between booming returns and sluggish results.

The bank reported 17 days with more than $100 million in trading gains, and 12 days with gains of nil to $25 million.

Morgan Stanley reported 11 days of trading losses in the quarter. Bank of America Corp reported only one day with trading losses above $100 million.

Goldman's second-quarter results come as the bank is working to comply with financial reform legislation that restricts so-called proprietary trading -- making market bets backed by its own capital.

The restrictions of the so-called Volcker Rule also curb banks' investments in private equity and hedge funds at 3 percent of a bank's total capital.

Goldman is weighing a number of options for its proprietary trading unit, including merging the employees into its asset management business or spinning off the division.

(Reporting by Joe Rauch; Editing by Lisa Von Ahn and John Wallace)