A gardener has told of his shock at coming face to face with an iguana as he trimmed his hedge.
Experts say iguanas would not survive a British winter outdoors
The 18in (46cm) reptile is now being cared for by an expert after Bryan Harris called the RSPCA to his home in Mumbles, Swansea.
Attempts are being made to trace the lizard's owner and to find out how it came to be on the loose.
Iguanas can grow more than 5ft long (1.5m) and RSPCA inspector Neil Manley said they do not make very good pets.
He said: "The man said he was cutting the hedge when he suddenly saw this great big lizard sat on it.
"Obviously he was surprised to see it and realised it wasn't a native species of the Mumbles.
"You get common lizards down this way but they are very small in comparison to large arboreal species like this iguana."
He added: "They don't make suitable pets to keep in captivity because they can grow very big and providing a suitable habitat is very difficult."
Terry Payne, chairman of the South Wales Reptile Society, is looking after the iguana on behalf of the RSPCA.
He said: "It's not that big at the moment. It's about 18 inches long and around two years old.
"It can grow up to one-and-a-half metres (5ft) long and develop a massive tail which can be quite dangerous because they sometimes lash out like a crocodile."
He said he suspected it had been abandoned as they cannot escape very easily.
Anyone with information about the iguana is urged to call the RSPCA on 0870 5555 999.