A federal grand jury indicted a former Houston assistant U.S. attorney yesterday on charges of trying to sell confidential information for $200,000 to the targets of a drug-smuggling and money-laundering investigation.

Frank Robin Jr. was indicted in Atlanta for alleged bribery and obstruction of justice, among the most serious accusations ever lodged against a federal prosecutor. Attorney General William French Smith played President Reagan a tape recording of the alleged bribe solicitation last fall to convince him to establish 12 new drug task forces, sources said.

The president was impressed and agreed to set up the special investigative units.

Robin was hired in July on a three-month contract to help investigate the financial activities of suspected drug dealers, tax attorneys and bankers on offshore islands, sources said.

He replaced an assistant U.S. attorney who had received an assassination threat. One witness had already been murdered during the course of the investigation, according to court records. Robin was supervising part of the case from Houston while other prosecutors were working in Atlanta.

The Washington Post reported last fall that in early August a caller who identified himself as "Charlie India"--for confidential informer--telephoned an investigator for one of the suspects under investigation. He offered to sell information outlining the government's investigative plans and to help to identify informers within the criminal group. The caller suggested that the conversation be taped because of its complexity. The suspect's attorney turned the tape over to the Justice Department, and attorneys in Washington supervising the case called other prosecutors, including Robin, in to hear it.

One of the supervisors thought at that time that the voice sounded like Robin's. Robin's work on the case ended Sept. 7, according to government court papers.

Robin's attorney, Michael J. Hinton, said yesterday that his client continues to insist that his isn't the voice on the tape. "We are very disappointed and Frank will plead not guilty," he said.

Justice Department officials said the last prosecutor to be charged with corruption was former federal prosecutor Donald E. Robinson Jr.

Robinson was acquitted of bribery and obstruction of justice charges in 1976 after he claimed he was trapped into taking $700 in bribes from undercover officers in the videotaped Washington "sting" investigation of the fencing of stolen goods.