Frank Al DeFilippo, formerly one of the closest aides of Gov. Marvin Mandel testified today that Mandel had a written agreement to join Tidewater Insurance Association, Inc. once his political carer had ended.

As in the last attempt to try Mandel - which was declared a mistrial on Dec. 7, DeFilippo was the opening witness in the government's attempt to prove that Mandel and five codefendants allegedly schemed to defraud the citizens of Maryland.

Despite opening statements by the prosecution that DeFilippo would "set the tone" of the government's evidence by showing that Mandel was promised "a piece of the action" of Tidewater, the aide gave testimony of the alleged agreement that followed his earlier and milder statement almost to the letter.

DeFilippo said that in 1970, before the gubernatorial race, Mandel had confided to him that Tidewater jointly owned by codefendants W. Dale Hess, Harry W. Rodgers III, and William A. Rodgers, had given him a letter of agreement with an option to join the firm once his political career had ended.

This agreement is part of a steady stream of favors that the government alleges was given to Mandel by the Tidewater executives and two other codefendants, Irvin Kovens and Ernest N. Cory Jr. In return, the government alleges that Mandel lobbied to pass legislation that would profit the business of the codefendants.

In the procession of four witnesses called today, the government made few alterations, appearing to stick with the strategy established during the last trial.

The last trial was aborted after the prosecution had presented its entire case when news of two outside attempts to tamper with the jury was leaked to the panel of jurors.

As before, wealthy Baltimore race track owner Nathan Cohen testified that Hess told him in 1971 that "were take care of the governor" by cutting Mandel in on business deals and then covering up the governor's involvement.

Cohen who with his family owns the Pimlico Race Course, was a partner in a ral estate venture that is central to Mandel's corruption trial. Cohen, Hess, and both the Rogers brothers went into partnership to purchase 200 acres of land known as Ray's Point on Maryland Eastern Shore.

Cohen said today that Hess and the Rodgers brothers gave Mandel a partnership in Ray's Point worth $45.000 and then concealed Mandel's participation by omitting his name from official records.