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The guide at Nemacolin Woodlands Resort in Farmington, Pennsylvania, about 60 miles south of Pittsburgh, was grabbed through a fence by a Himalayan bear, pulled toward it, and bit on the arm, according to the resort.
The female employee was flown by helicopter to a local hospital and was in critical condition on Sunday, according to Ruby Memorial Medical Center in Morgantown, West Virginia.
"We deeply regret this incident," Maggie Hardy Knox, president of Nemacolin Woodlands Resort, said in a statement. "Our thoughts are with our injured associate, our staff and guests as we focus on ensuring they receive the finest medical attention and counseling."
The resort said it would provide counseling to guests and staff who witnessed the attack.
The bear has been at the wildlife reserve for nine years, it said.
The organization advertises one-hour tours where you can feed various livestock before viewing wolves, tigers, lions and buffalo. "Don’t forget the bear enclosure, where we hear the bears enjoy eating marshmallows," the adventure advertises.
The bear has not been put down, the Pennsylvania Game Commission said.
There are two different types of Himalayan bears, both black and brown bears, but the resort did not specify which attacked the staff member. Both are large bears, standing over 5 feet tall and weighing in excess of 200 pounds.
The bears' natural habitat is in the Himalayan Mountains, as their name suggests, from northern Afghanistan through Pakistan and into northern India, Nepal and western China.
ABC News' Josh Hoyos and Olivia Rubin contributed to this report.