American Express Co. (AXP) said Thursday in a regulatory filing that U.S. borrowers at least a month behind their card payments stood at 4.1% in September, unchanged from August.

For the quarter that ended Sept. 30, the 30-day delinquency rate was 4.1%, according to preliminary data, down from 4.4% in the second quarter.

The 30-day delinquency rate, a key gauge of future losses, is important because higher delinquencies force issuers to squirrel away capital to reserve for potential losses; ultimately, companies must write off loans if customers can't pay. A slowdown in the delinquency rate is noteworthy because it comes at a time when seasonal factors - such as good behavior on the part of borrowers fueled by tax refund checks - are behind the card industry.

This monthly report card on the performance of credit card loans, including those packaged into bonds, comes amid heightened scrutiny around credit as losses stemming from souring card loans pile up.

AmEx wrote off 8.4% of its card loans last month, according to the regulatory filing. In August, the company wrote off 9% of its U.S. card loans. This write-off rate is annualized.

For the third quarter, the company wrote off 8.9% of its U.S. card loans, according to preliminary data, down from 10% in the second quarter.

The company's shares recently traded at $34.79, down 28 cents, or 0.80%. The stock has traded in the range of $9.71 and $36.50 in the last 52 weeks.

An AmEx spokeswoman wasn't immediately available to comment on the company's credit card loans performance.

Like other card issuers, AmEx has been hurt by cutbacks in spending and customers who are falling behind on their bills during the current economic slump. Unlike other card companies, AmEx both issues cards and processes transactions. It issues both charge cards requiring a monthly payoff and credit cards on which customers can carry a balance.

-By Aparajita Saha-Bubna, Dow Jones Newswires; 617-654-6729; aparajita.saha-bubna@dowjones.com