Zookeepers say they last saw 13-year-old Butti (BOO'-tee) on Monday afternoon in The Loft, a hands-on exhibit where visitors can handle Butti and his brother Tutti.
Officials say someone might have walked off with the Indian Star tortoise -- he's the size of a grapefruit and weighs about a pound.
Zookeepers are most concerned about Butti's health because he requires a special diet of certain vegetables.
"This type of tortoise is not uncommon in the pet trade," said Katie Borremans, the zoo's public relations manager.
"We think someone probably thought the tortoise looked interesting and is planning to keep him as a pet, but we're worried Butti won't receive proper care."
Indian Star tortoises typically live 10 to 15 years.
Butti has a history of upper respiratory infections. Without his caretakers, he might develop gastrointestinal problems, osteoporosis or kidney failure.
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