NEW YORK — After giving the stock market a big gain during August, investors still worried about the economy backtracked the final day of the month.

Stocks fell in light trading yesterday after a 6.7 percent plunge in China's main stock market sent a wave of selling worldwide and added to concerns that stocks have rocketed too high, too fast since hitting 12-year lows in March.

The Standard & Poor's 500 index rose 3.4 percent in August for its sixth straight monthly gain, advancing despite some periodic choppy trading as investors fretted about an economic recovery. It is up 50.9 percent since early March, the best six-month run since 1938.