Legg Mason, home for a few more months to the once-legendary mutual fund manager Bill Miller, filed regulatory paperwork with the Securities and Exchange Commission to bring to market the company’s first ETF, a short-dated fixed-income fund that looks like a clone of Pimco’s $1.58 billion fund, MINT.

The new fund, Legg Mason Western Asset Ultra-Short Duration ETF, is an actively managed bond fund that will own short-term, investment-grade fixed-income securities that include corporate debt securities, bank obligations, mortgage-backed securities, as well as securities issued by the U.S. government and foreign governments.

The expected duration date for the proposed ETF will be a year or less, according to the filing. Its composition and its duration make the new Legg Mason fund look at lot like the Pimco Enhanced Short Maturity Strategy Fund (NYSEArca: MINT), which also invests in short-term investment-grade debt. MINT’s effective duration is 0.99 years, according to Pimco’s website.

Baltimore-based Legg Mason didn’t provide the planned ETF’s annual expense ratio or its ticker. MINT costs investors 0.35 percent a year.

Entering the ETF space is a big move for Legg Mason, which filed for exemptive relief to bring to market actively managed ETFs in 2010. Legg Mason is one of a number of several mutual fund companies, including Alliance Bernstein, Janus and Dreyfus, among others, that have been laying the groundwork to market actively managed ETFs. It appears to be the first among them to actually put a fund into registration.

In a way, it’s fitting that Legg Mason is putting its first ETF into registration at a time when one of its stars is heading off into the sunset.

Earlier this month, the firm said Bill Miller, who managed the company’s $2.8 billion Legg Mason Capital Management Value Trust fund, is stepping down in April of next year.

After beating the market for 15 years in a row through 2005, the fund has trailed the S&P 500 Index in five of the past six years. Its assets have dwindled over that period by almost $18 billion, according to the Wall Street Journal.

Over that period, assets in ETFs have ballooned and now stand at more than $1 trillion. Indeed, even as markets have swooned in the wake of the market meltdown of 2008-2009, ETFs have, for the most part, kept gathering assets. However, with the exception of MINT and a few other active ETF strategies, less than 1 percent of ETF assets are in active funds.

In contrast to passively managed ETFs, active ETFs don’t track an index and are dependent upon the expertise of the fund’s investment advisors for performance.

Unlike mutual funds, ETFs—active as well as passive—are transparent and they must disclose their holdings every day. Also, unlike mutual funds, ETFs—even active ETFs—offer investors the advantage of intraday trading. That means, among other things, they can more readily sell off their holdings if necessary.