"With just a few exceptions, overall asset flows into active ETFs have been not great," said Robert Goldsborough, ETF analyst for Chicago-based Morningstar. "By the same token, not a lot of ETF providers have come up with actively managed ETFs."

In June, Atlanta-based Invesco Ltd. said it would liquidate two actively managed ETFs -- PowerShares Active Alpha Multi-Cap Fund and PowerShares Active AlphaQ Fund. Columbia Management Investment Advisors LLC, a unit of Minneapolis-based Ameriprise Financial Inc., in April said it had agreed to acquire actively managed ETF firm Grail Advisors LLC. Columbia's five actively managed ETFs have about $23 million in assets, according to data compiled by Bloomberg.

The Pimco Total Return ETF may benefit, compared with offerings from smaller firms such as Grail, from Pimco's existing brand, distribution and marketing, Goldsborough said.

"There's a lot of inherent advantages to what Bill Gross has," he said. "There's an untested aspect to a smaller firm that isn't there with a bigger firm like Pimco."

The Pimco Enhanced Short Maturity Strategy Fund is the largest actively managed ETF. The fund invests primarily in short-duration investment-grade securities and is managed by Jerome Schneider, and has about $1.4 billion in assets. The fund returned about 1 percent for the year ended Feb. 29, Bloomberg data show.

Investors who currently hold higher-expense share classes of the traditional fund may consider switching to the ETF for the cost savings, said David Hallman, vice president of investment research and management for Newport Beach, California-based United Capital, which with its affiliates oversees about $16 billion.

The ETF may appeal to "mom and pop" investors, Gross said. The fund's annual expenses are lower than those of the A, C and D share classes of the flagship fund, where costs range from 0.75 percent to 1.6 percent.

"The challenge is obvious," Gross said. "We could fall flat on our face or we could roar like a lion in a year or two or three and become the largest ETF."

Bond ETFs may be more likely to trade at substantial premiums or discounts to their net asset values than stock ETFs, said Michelle Knight, chief economist and managing director of fixed income for Boston-based Silver Bridge Capital Management, which oversees about $3.2 billion for families and institutions.

In January, taxable bond ETFs traded with an average monthly premium of 23 basis points, compared with an average monthly discount of 4 basis points for U.S. stock ETFs, according to Morningstar. The average monthly premium for taxable bond ETFs during 2008 widened to as much as 1.87 percent.