The fund will invest at least 80% of its assets in, or derive at least 80% of its income from, securities that are exempt from regular federal income tax and that do not subject the investors to federal alternative minimum tax. The fund invests primarily in debt securities rated AA- or better or Aa3 or better, or unrated but determined by the fund's investment adviser to be of equivalent quality. Its aggregate portfolio will have a dollar-weighted average effective maturity no greater than three years.