Dolly, the world's first cloned sheep, has been euthanised at the age of six after being diagnosed with progressive lung disease, the Edinburgh-based Roslin Institute said on Friday.
"Sheep can live to 11 or 12 years of age and lung infections are common in older sheep, particularly those housed inside," said Dr Harry Griffin, head of the institute.
"A full post-mortem is being conducted and we will report any significant findings."
Dolly, named after the singer Dolly Parton, was the first mammal to be cloned with DNA taken from an adult cell.
Her birth -- one of the most significant scientific breakthroughs of the 1990s -- triggered a debate about the ethics of cloning.
Dolly's body has been promised to the National Museum of Scotland and will be put on display in Edinburgh, a spokesman for the Roslin Institute was quoted as saying.