CINCINNATI (AP) _ The U.S. Postal Inspection Service said today it is investigating allegations that Dun & Bradstreet Corp. defrauded customers who buy its credit information.

Joseph Clark, the Cincinnati postal inspector handling the investigation, said he has questioned several Dun & Bradstreet employees, but declined to give details.

''We're in a position where we wouldn't deny it, but I can't comment further than that on the specifics of the investigation,'' Clark said.

Clark said the investigation developed from allegations in a suit filed in U.S. District Court here by a Dun & Bradstreet salesman who alleged that the company engaged in misrepresentations to induce customers to buy more credit- reporting services than they needed.

The company has denied the allegations and said it has begun an internal investigation.

The Wall Street Journal reported today that the investigation is focusing on the Cincinnati office of Dun & Bradstreet, one of the nation's biggest credit-reporting services.

U.S. Postal Service investigators have requested internal Dun & Bradstreet documents and the company is complying, said Reid Gearhart, a spokesman at Dun & Bradstreet's New York City headquarters.

''I believe that's being done right now,'' Gearhart said.

Dun & Bradstreet salesmen William Lewis and Gary J. Mertz told the Wall Street Journal that Clark interviewed them and several of their colleagues.

Lewis and Mertz told the newspaper that Dun & Bradstreet managers have for years encouraged salesmen to misrepresent to customers how much of the credit data they bought was needed. Customers who relied on those representations would buy more credit information than needed, Lewis and Mertz told the Journal.

Dun & Bradstreet has said it settled with 11 Philadelphia-area customers who claimed they bought more services than needed. Those customers complained to Dun & Bradstreet after receiving an anonymous letter, apparently from inside the company, the Wall Street Journal reported.