AmEx (AXP) shares dropped to their lowest levels since mid-October on Thursday after the credit card company announced that its exclusivity deal with wholesale club retailer Costco is set to expire at the end of March 2016. The market reacted swiftly and sharply to the prospect of Costco no longer accepting AmEx cards. AmEx is currently the only credit card accepted by the retailer, which is one of the largest U.S. retailers with nearly 470 stores across the country.

The credit card company’s shares dropped to around $80 in early trading and were recently trading down by about 6.7%, at $80.88. The steep decline erased roughly $5.9 billion from the payment card giant’s market value, which is still nearly $88 billion.

Costco previously dropped AmEx as its exclusive credit card issuer in Canada and Bloomberg reported last fall that the retailer was considering making the same move in the U.S. The retailer negotiated a deal to partner with Capital One Financial Corp. (COF) and MasterCard (MA) in Canada. Shares of Capital One and MasterCard were each up roughly 3% on Thursday.

In a Thursday morning earnings call, AmEx said losing the Costco contract would drag down its earnings and revenue this year and in 2016. The company said it now expects earnings to be flat this year after analysts projected 10% earnings growth, based on polling by Thomson Reuters. AmEx expects earnings growth to return next year.

AmEx and Costco had been engaged in negotiations to extend their agreement in the U.S., but the two sides were unable to reach a deal, AmEx CEO Kenneth Chenault said in a statement. The chief executive added that his company will instead “focus on opportunities in other parts of our business where we see significant potential for growth and attractive returns over the moderate to long term.”

AmEx shares are down 13% on the year and the company announced last month that it plans to cut more than 4,000 jobs this year.
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