To settle a stockholder's lawsuit, the managers of Washington Mutual Investors Fund have agreed to refund $200,000 in management fees and to reduce their charges by $500,000 over the next 10 years.

The fund also has agreed to hire an independent expert to evaluate the management fees charged by WMIF Management Corp., which is owned by Johnston, Lemon & co., the Washington stockbrokers.

The actions are part of a settlement offer made to terminate a lawsuit claiming that management costs of the mutual fund were excessive.

The lawsuit was filed four years ago by E. K. Zewdski, trustee for an estate that owes more than $200,000 worth of stock in Washington Mutual Investors.

Zewadski sued WMIF Management, Johnston, Lemon-which organized the fund and set up the management company-and all the WMIF board members who are not affiliated with Johnston, Lemon or the management company.

The lawsuit challenged not only the size of the management fees charged by the Johnston, Lemon subsidiary but also the relationship between Johnston, Lemon and the mutual fund. Zewadski accused the outside directors of WMIF of breaching their duties to stockholders by not questioning links between the two companies.

In the proposed settlement, the directors, the management company and Johnston, Lemon continue to deny any wrongdoing in operation of the $300-million-a-year fund.

The settlement will reduce by $50,000 a year the management fees the fund pays; the fees run about $700,000 a year, a WMIF executive said.

The reduction is retroactive to May 1, 1975, about the time Zewadski's lawsuit was filed. Under terms of the proposed settlement, the management firm will pay back $50,000 a year for the past four years and use a lower fee schedule for the next 10 years.

Two-thirds of the repaid management fees will go to attorneys for Zewadski, who was represented by Alan S. Novins of Washington. After deducting Novins' fees-up to a maximum of $133,000-the settlement will have little impact on the earnings of the mutual fund, a spokesman said.

In addition to cutting management charges directly, the settlement calls for hiring an independent lawyer to negotiate the contract for management of the fund.

Although no contract is supposed to be negotiated on an arms-length basis, the same lawyer has represented the fund and the management in past dealings, the lawsuit charged.

The independent counsel will have authority to hire additional outside specialists to evaluate the management fees and advise the independent board members.

The six outside directors of WMIF who were named as defendants in the lawsuit will not pay any of the costs of the settlement, an attorney for WMIF said.