William G. Hessler, a former vice president of Riggs National Bank, pleaded guilty yesterday to failing to report a $450,000 cash deposit that prosecutors said was part of a scheme to launder money from cocaine sales.

Hessler, 48, once described by his lawyer as a "very trusting . . . gentleman banker," was the 14th person to be convicted in the case, which involved a major cocaine distribution ring that smuggled the drug from Colombia and sold it in many parts of the United States.

Those convicted previously include former Washington lobbyist Fred B. Black Jr.; Marcos Cadavid, a Colombian cocaine wholesaler, and Lawrence G. Strickland Jr., Hessler's brother-in-law, who was described in court as the leader of the cocaine ring.

Yesterday, S. Edgar Wilhite, Hessler's attorney, told federal Judge Thomas F. Hogan that Hessler had no knowledge that the "ultimate source" of the large cash deposit was trafficking in drugs or was Strickland. But Hessler acknowledged that he did not file a currency transaction report with the Internal Revenue Service, which is required for all cash desposits over $10,000.

In a trial last spring, which ended in a hung jury, witnesses said Black carried the money in a suitcase in September 1979 into the Riggs branch at the Watergate, where Hessler was manager.

Black, 71, who is now serving a seven-year prison sentence, was convicted at that trial on three counts of income tax evasion. He previously was convicted of conspiracy to import and distribute cocaine.

Yesterday, Assistant U.S. Attorney Roger M. Adelman said the government would drop 13 counts still pending against Hessler after he is sentenced on Feb. 20.

Hessler, who lives in Rockville and has been supporting himself as a house painter, faces a maximum penalty of five years in prison and $500,000 in fines. Hogan allowed him to remain free on personal bond.

After the guilty plea, U.S. Attorney Joseph E. diGenova noted that the case, which began with an indictment in December 1983, was the first major drug case brought in Washington under the Organized Crime Drug Enforcement Task Force Program, started by the Reagan administration in 1982.

"We are now moving into the second phase of this investigation," diGenova said, "which involves other substantial cases of narcotics money laundering in the United States and other countries."

DiGenova added that aside from Hessler, who was fired by the bank three years ago, "there is no evidence whatsoever of any other involvement at Riggs" with the money laundering. He said Riggs officials had "cooperated fully" with federal investigators.

According to the charge to which Hessler pleaded guilty yesterday, the $450,000 deposit was "part of a pattern of illegal activity" involving over $900,000 in unreported cash deposits by Black during 1979 and 1980.

At the trial, Strickland, 34, who testified as a key prosecution witness, said he had delivered the cash to Black in his Watergate apartment for deposit in Riggs accounts controlled by Black.

The indictment said Hessler received $40,000 in bribes from Black and made fraudulent loans to him on which Riggs lost $130,000. Hessler testified that the money he received was lent and that he followed bank policies at all times.