The largest U.S.-listed municipal bond exchange traded fund has started trading at a discount to its net asset value for the first time in five months.

The iShares S&P National AMT-Free Muni Bonds Fund (NYSEArca: MUB) first started trading at a discount to its NAV Thursday and continues to trade at a discount of around 0.30%. The ETF has already fallen below its 50-day exponential moving average and is now testing its 200-day supporting level.

ETFs hold a basket of securities to mimic an underlying benchmark. When the ETF’s price is lower than the NAV, the ETF is said to be at a “discount” – the ETF is valued less than the fund’s overall holdings. If the ETF’s price is above the NAV, the ETF is said to trade at a “premium” – the ETF is trading higher than what the underlying holdings are worth. [Three Things to Remember About ETF Premiums and Discounts]

After peaking in February, the muni bond ETF has fallen to its lowest level since December.

The improving labor market prompted to the Federal Reserve to provide a more optimistic outlook for the economy, which depressed U.S. Treasuries, along with municipal bonds, according to a Bloomberg report.

In the week ended March 9, investors dumped $10.4 billion in fixed-rate munis, the most since November. As muni prices declined, yields shot up 0.17% for the week, the largest rise since October.

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