D.C. Convention Center officials, under questioning from congressmen yesterday about their hiring of a company with business ties to South Africa, testified that the firm had no connection to South Africa when the center contract was signed.

But a top center official later acknowledged in an interview that a subsidiary of Delaware North Companies Inc., parent company of the firm hired by the center, had actually been operating in South Africa since 1981, more than a year before the center contract was signed. He said center officials had testified incorrectly yesterday before Rep. Julian Dixon (D-Calif.), chairman of the House Appropriations subcommittee on the District.

Dixon, a leader of the congressional effort to prohibit American companies from operating in South Africa because of its apartheid policies, had closely questioned the officials about their failure to consider Delaware North's South African operations.

Center officials responded after some discussion that the company had no such operation when the contract was signed.

Michael Rogers, deputy convention center general manager, said after the hearing that center officials had been told by Delaware North officials on two occasions, including as recently as yesterday, that the South African contract had been signed in October 1982, about a month after the food service contract was awarded to Sportservice Corp., a Delaware North subsidiary.

A brochure put out by Delaware North said it began operations at Jan Smuts Airport at Johannesburg in 1981. Rogers said he was unaware of that information.

He subsequently said that center officials had been given the wrong date. "I think it was just a miscommunication," Rogers said.

Dixon, who left late yesterday for California, could not be reached for comment.