The fund invests at least 80% of its assets in a diversified portfolio of debt securities whose interest is, in the opinion of bond counsel for the issuer at the time of issuance, exempt from federal income tax ("Municipal Bonds"). Municipal Bonds generally are issued by or on behalf of states and local governments and their agencies, authorities and other instrumentalities. It may only invest in U.S. dollar-denominated investment grade debt securities, rated Baa or higher.