BlackRock has thrown a new type of product into the mix in the rapidly evolving municipal bond exchange-traded fund industry.

It has introduced six municipal bond ETFs, which it said on Friday are the first of their kind. Unlike other muni ETFs, they liquidate at a target date. The liquidation feature makes these funds similar to unit investment trusts or even municipal bonds themselves.

The products' introduction brings the number of muni ETFs to at least 26. The product was nonexistent before late 2007, but the industry managed $5.78 billion in this category at Nov. 30, 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. And like most ETFs, the new BlackRock funds try to ape the performance of a benchmark index.

The difference is that, instead of continuing indefinitely, the funds close down and return all their money to shareholders at a target date, in the new funds' case, each year from 2012 to 2017.

Matt Tucker, managing director of U.S. fixed-income strategy at BlackRock, said in a press release that the new series provides a "set of tools for helping to meet municipal bond needs such as building and maintaining municipal bond ladders."

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