A rogue trader at MF Global Ltd. rang up $141.5 million in losses on the broker’s account this week, the company said Thursday, a black eye that cost the company almost a fifth of its market value.

The Bermuda-based broker said on Wednesday morning it discovered Evan Dooley, a trader at the company’s Memphis, Tenn., branch, trading wheat contracts in amounts that exceeded how much he was allowed to trade.

MF Global said it fired Mr. Dooley and liquidated the wheat contracts, which led to a $141.5 million loss. An entry-order system that should have blocked the trades failed, the company said.

Attempts to locate an Evan Dooley in the Memphis area for comment were unsuccessful.

MF Global brokers options and futures contracts for 130,000 clients at 70 exchanges like the Chicago Mercantile Exchange, the Chicago Board of Trade and the New York Mercantile Exchange. The company said the trading loss does not threaten the $19.5 billion clients have in their accounts.