In what it's calling the longest-term bond exchange-traded fund available to investors, State Street Global Advisors on Wednesday launched the SPDR Barclays Capital Long Term Credit Bond ETF (NYSE Arca: LWC).

The new ETF is designed to provide access to investment-grade corporate and noncorporate bonds with maturities of 10 years or more. It comes with an expense ratio of 0.15%. 

Entering 2009, LWC's index included 965 issues with an average dollar-weighted maturity of 24.39 years.

SSgA already has another similar fund that invests in Treasuries (NYSE: TLO). But its average maturity is 18.41 years and its 30-day SEC yield is around 3.84%.

By comparison, the Vanguard Long-Term Bond ETF (NYSE: BLV) had an average maturity of 20.8 years heading into February.

As the new SPDR's closest competition, BLV also might provide a snapshot of how including corporates along with government-related issues can help boost yields in today's market. It has a yield of around 5.5% and includes some foreign bonds in its mix as well.

BLV has attracted some $2.9 billion in assets since launching nearly two years ago. It also will challenge LWC in terms of pricing with an expense ratio of 0.11%. 

In all, six other long-term bond ETFs are already on the market. But those focus on Treasuries and don't take as diversified of an approach as either LWC or BLV.

"Demand for access to high quality, long-term credit bonds is on the rise as investors search for alternatives to U.S. Treasuries that will improve the yield on their fixed income portfolios," said Anthony Rochte, senior managing director at State Street Global Advisors, in a statement.

Given the state of markets lately, demand has been strong for long-term bond funds. Typically, longer-termed issues stand to gain more than those with shorter-terms in periods of low-interest rates and bearish stock markets. 

"The SPDR Barclays Capital Long Term Credit Bond ETF is the longest-maturity credit bond ETF available to investors and a key addition to our growing family of fixed income SPDRs, which have attracted more than $800 million in net inflows in just the first two months of 2009," said Rochte.