NEW YORK ( TheStreet ) -- Investors have been pouring into ultra-short ETFs, including PIMCO Enhanced Short Maturity ( MINT). In September, the PIMCO fund emerged as the largest actively managed ETF, according to Morningstar. With $3.8 billion in assets, the ultra-short bond fund took the top spot away from PIMCO Total Return ETF ( BOND), an intermediate-term fund that has $3.7 billion in assets. Run by bond star Bill Gross, the intermediate ETF is similar to PIMCO Total Return ( PTTRX), the largest bond mutual fund. The move to short-term funds accelerated as investors became concerned about rising rates. When rates rise, most bonds tend to fall, but short-term issues are relatively resilient. The migration to ultra-short funds represents a sharp change from the trends of recent years. As the financial crisis unfolded in 2008, investors began dumping stocks and racing to bonds. At first, ultra-short funds attracted little attention. Instead, investors focused on intermediate-term funds, which offer higher yields. In May this year, interest rates spiked. For the month, PIMCO Total Return ETF lost 2.2%, while the PIMCO ultra-short ETF only declined 0.1%.The losses caused investors to scramble. From June through the first week in November, shareholders pulled $1.2 billion out of PIMCO Total Return ETF and invested $857 million into the ultra-short ETF, according to IndexUniverse.com. Along with PIMCO, other fund companies have been introducing ultra-short ETFs and attracting assets. Competitors include FlexShares Ready Access Variable Income ( RAVI), Guggenheim Enhanced Short Duration Bond ( GSY) and SPDR SSgA Ultra Short-Term Bond ( ULST). For many investors, the ultra-short funds offer a way to get a bit more yield than money-market funds offer -- without taking on much more risk. According to Crane Data, the average money market yields only 0.02%. In contrast, the Guggenheim ultra-short ETF yields 0.92%, while the SPDR ETF yields 0.54%. Yields on money markets are being held down by new regulations imposed since the financial crisis. Under the rules, money markets are required to maintain average weighted maturities of 60 days. In contrast, the ultra-short funds often own securities with maturities ranging from a few days to 7 years. In the current market, bonds with slightly longer maturities deliver substantially higher yields. While 3-month Treasury bills yield 0.08%, 5-year Treasuries yield 1.42%.