By Colum Murphy 

SHANGHAI--BYD Co., the Chinese electric-car maker backed by U.S. investor Warren Buffett, saw its shares fall more than 40% on Thursday amid a worsening outlook for the Chinese car market and lower oil prices.

The reason for the sharp drop wasn't immediately clear. A BYD spokeswoman said the company's operations remained normal and that it didn't see an explanation. "Everything's well," she said. "I think it's market behavior. We'll pay attention to it."

BYD's shares gained a bit back later Thursday, and were trading late in the session at 25.20 Hong Kong dollars, down 28%.

A 50% drop in oil prices over the past six months has pressured green stocks in a number of areas.

The company also has been beset by rising competition for its conventional, gasoline-powered cars. In October it reported a third-quarter profit drop of 26% and said it expects this year's profit to fall by up to 22%. Auto sales growth in China has slowed in recent months amid a broader drop in China's economic momentum.

Mr. Buffett's investment vehicle, Berkshire Hathaway Inc., owned a nearly 25% stake in BYD as of Sept. 30, according to FactSet. The spokeswoman said the company didn't think the drop was related to Berkshire Hathaway.

Write to Colum Murphy at colum.murphy@wsj.com

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