"Investors continue to use ETFs across the board to express their market views," Gamba said. "We had a negative June followed by a very, very positive July, which was close to $45 billion in inflows, and now we had $15 billion in outflows in August."
(Related: As market fell, investors moved toward risk)
Though the $15 billion represents the largest ETF one-month outflow on record, it is only about 1 percent of total ETF assets, he said. Outflows to total assets in June 2010 were larger on a relative basis, at 2.2 percent, he added.
Both equity and fixed-income ETFs saw money depart last month, while research showed bright spots in Europe and short-term fixed income. U.S.-focused equity ETFs took the majority of outflows, at $9 billion.
(Related: Where did earnings go? Profit outlook gets gloomy)
Despite the weak performance, the ETF market has been growing 10 percent to 12 percent this year, Gamba said, and the overall movement has been strongly positive. With more investors using ETFs in general, he said, "it's been more volatile."
New growth in the asset class is largely attributed to insurers, global money managers and international investors, Gamba added.