The former assistant general manager of Bethlehem Steel's Baltimore shipyard has pleaded guilty to a charge that he defrauded a shipping company out of $50,000.

Thomas A. LaMonica, who was dismissed by Bethlehem in 1976, faces a possible sentence of five years and a fine of $1,000, for the charge which grew out of LaMonica's participation in a complicated scheme of bribery by the giant steel company. Sentencing was scheduled for Oct. 28.

Bethlehem pleaded guilty last month to paying more than $400,000 in bribes to shipowners' agents to win lucrative ship repair contracts for its shipbuilding yards and said that it accepted corporate responsibility for the illegal acts uncovered in an investigation headed by Assistant U.S. Attorney Jane W. Parver. Although the corporation faces a possible fine of up to $518,000, no corporate official was indicted for the bribery scheme. The scheme lasted until 1976 and involved raising almost $2 million for a fund that was used to pay bribes.

In 1977, Bethlehem charged two former employes, LaMonica and Clifford R. Wise, former manager of Bethlehem's ship repair sales operation, with embezzlement in a civil suit. That trial is scheduled for November. Wise was granted immunity from criminal prosecution resulting from the bribery investigation.

LaMonica, of Parkton, Md., pleaded guilty Wednesday to engaging in a scheme to obtain $50,000 by fraud from Victory Carriers Inc., a New York shipping concern that was a customer at the Key Highway shipyard in Baltimore. According to the criminal information, LaMonica knew that Bethlehem had already paid a Victory employe $30,000 to obtain repair contracts for two of the company's ships.

LaMonica and Victory's representative entered into an agreement to submit phony invoices that were paid by Victory. The two then split the proceeds.The Victory representative was not named in the indictment.

"LaMonica, knowing that Bethlehem Steel had already corrupted the Victory Carrier's representative for corporate purposes found he was able to enter into a separate and distinct corruption agreement with that same agent for his personal benefit."