Some Maryland customers of American Telephone & Telegraph Co. are getting a nasty surprise in this month's phone bill -- charges of six times the norm for some long-distance calls.

According to Chesapeake & Potomac Telephone Co., a programming error in C&P;'s billing software caused 200,831 long-distance calls placed by Maryland residents to be billed at rates up to six times higher than usual.

C&P;, which does billing for the long-distance companies, corrected the problem May 17. But by that time the programming error had already miscalculated the cost of almost half a million calls from Washington, Maryland and Virginia. Those miscalculations are showing up in this month's C&P; phone bills.

Jeanine Smetana, a C&P; spokeswoman, said that C&P; already knows who was overcharged and plans to issue credits on next month's bill. The credits will be issued automatically. As a result, she said, it isn't necessary for customers to contact C&P; about this month's long-distance billing errors.

But customers who don't want to wait a month for credit from C&P; can elect to deduct the charges for the improperly billed calls from this month's payment, said Jim McGann, an AT&T; spokesman.

Those calls will be charged at the correct rates in next month's bills, he said.

According to Mr. McGann, it is easy to single out miscalculated calls: Look in the column that lists the duration of long-distance calls. The errant calls are timed to a fraction of a minute, such as "5.2 minutes" or "14.1 minutes." Normal calls are billed by whole minutes.

C&P; said billing errors were limited to some residential customers who subscribe to AT&T;'s discount calling plans, such as "Reach Out America."

Intrastate calls and customers of other long-distance carriers were not affected, C&P; said.

Michel Daley, a C&P; spokesman in Washington, said that 462,000 calls in the District, Virginia and Maryland were miscalculated before the programming error was caught.