Chipotle Mexican Grill Inc. posted its first loss as a public company Tuesday, with its sales tumbling 23% following a series of food-illness incidents.

Chipotle is looking to rebuild its image after a number of disease outbreaks tied to its food damaged the restaurant's sales, stock price and reputation.

"As our sales are on a gradual path to recovery, we remain focused on our mission of changing the way people think about and eat fast food," said Steve Ells, co-chief executive of the company. "The best approach to re-building our business is to proudly serve safe and delicious food...which is exactly what we will continue to do."

Shares of the company, down 42% from its high in August, fell 3.1% to $432 after hours.

The company reported that same-store sales fell 29.7% during its first quarter, as same-restaurant transactions slid 21.1%. The declines were partially offset by the opening of 58 new locations.

Expenses also jumped during the period, with restaurant operating margins sinking to 6.8% from 27.5% a year earlier. The change was due in part to "higher than usual marketing and promotional costs and food testing and waste costs."

After government officials declared an end to the largest of the outbreaks in February, Chipotle unveiled a promotional campaign that offered free burritos, chips and salsa or guacamole. Chipotle has said the giveaways have helped sales improve, but some analysts have expressed concerns that the large-scale promotions may undermine Chipotle's recovery by cutting into profit.

Over all, for the first quarter, Chipotle reported a loss of $26.4 million, or 88 cents a share, down from $122.6 million, or $3.88 a share, a year earlier.

Revenue declined to $834.5 million from $1.09 billion.

Analysts polled by Thomson Reuters expected a per-share loss of 95 cents and revenue of $868 million. For the first quarter, Chipotle had said it expected to report a loss of $1 a share, compared with a year-earlier profit of $3.88 a share.

Write to Ezequiel Minaya at ezequiel.minaya@wsj.com

(END) Dow Jones Newswires

April 26, 2016 17:45 ET (21:45 GMT)

Copyright (c) 2016 Dow Jones & Company, Inc.
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