BlackRock Friday threw a new type of product into the mix in the rapidly growing and evolving municipal bond exchange-traded fund industry.

The company launched six municipal bond ETFs, which it said are the first of their kind. They are similar to other muni ETFs except they liquidate at a target date.

The liquidation feature makes these funds similar to unit investment trusts or even municipal bonds themselves.

The launch brings the number of muni ETFs to at least 26. Nonexistent until late 2007, the industry managed $5.78 billion at the end of November, according to the Investment Company Institute.

The first wave of municipal ETFs followed a standard format: a fund would raise money from investors and buy a basket of muni bonds, trying to replicate the performance of a target index.

Pacific Investment Management Co. last month introduced a municipal ETF that tries to outperform the market rather than mimic an index.

On Friday, BlackRock’s iShares brand launched six ETFs that liquidate at a planned date.

Like most ETFs, these funds try to ape the performance of a benchmark index. The only difference is that instead of continuing indefinitely, the funds close down and return all the money to shareholders at the target date.

For instance, the iShares 2017 S&P AMT-Free Municipal Series, which launched Friday with $5 million in assets, will try to match a Standard & Poor’s index tracking 983 investment-grade bonds maturing in 2017.

The fund will pay monthly dividends like any exchange-traded fund. As the bonds the fund owns mature, the ETF will stash the money in money market funds, short-term notes, commercial paper or other cash-like instruments.

At the end of August 2017, the fund will distribute all the money to shareholders and terminate.

“Investors can now obtain targeted exposure to the municipal yield curve with iShares ETFs that also offer both liquidity and diversification,” said Matt Tucker, managing director of U.S. fixed-income strategy at BlackRock. “The new series provides advisers and investors with an invaluable new set of tools for helping to meet municipal bond needs such as building and maintaining municipal bond ladders.”

Laddering is an investment strategy that involves building a portfolio with a range of maturities, continually buying long-term bonds with the cash collected on maturing bonds.

BlackRock said it plans to offer new funds in this series continually.

The six funds in the new iShares line carry liquidation dates from 2012 through 2017. Every fund launched with $5 million and is managed by Lee Sterne and Joel Silva.

BlackRock, which bought iShares from Barclays Global Investors in December, now runs 10 muni ETFs with more than $2 billion in assets. The company’s suite includes the biggest muni ETF, iShares National AMT-Free Municipal Bond Fund, with $1.6 billion in assets.

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