Boston-based State Street Corp. has been slapped with a class action lawsuit on behalf of some shareholders.

The lawsuit was filed on behalf of purchasers of the SSgA Yield Plus Fund, by the law firm of Schiffrin Barroway Topaz & Kessle, based in Pennsylvania.

The complaint alleges the company failed to disclose that a majority of the fund's assets were invested in mortgage-backed and mortgage-related securities; that the "risk managment" disclosed by State Street was insufficient because it did not warn of the extent to which the fund was invested in mortgage-backed and mortgage-related securities, nor of the actual risks of investing in the fund; and that, as a result of the foregoing, the fund's registration statement was false and misleading at all relevant times.

The lawsuit charges State Street and certain related entities with violations of the Securities Act of 1933. State Street Global Advisors is the investment advisor to the entire group of mutual funds under the State Street name.

The SSgA fund is a mutual fund promoted as having an objective of seeking "high current income and liquidity," according to the lawsuit.

From July 2006 through July 10, 2007, the fund's shares traded at or above $9.88 per share. Thereafter, the suit alleges, the value of the fund's shares began to plummet. By April, the value of the share's had fallen to as low as $6.60.