Aquarium staff have installed CCTV after they say a visitor attempted to make off with a stingray.
Management at the SeaQuarium in Weston-super-Mare, Somerset, said a customer was spotted reaching in to an enclosure in order to grab a freshwater stingray.
They said the attempted theft was witnessed by another visitor, who informed staff. However, they were unable to trace the suspect, prompting the introduction of security cameras to protect the endangered species.
Manager Christopher Donkin said: "(There are) warning signs to not touch the freshwater stingrays in the Rainforest River Zone, as these are babies and small, (and) they can have a nasty sting in their tails. Anyone trying to take one home would be in for a bit of a surprise.
"It's such a shame, we are continuously creating exhibits which are open-aired, letting our visitors get even closer, but you have to draw the line somewhere."
Aquarium staff have also increased their walk rounds in a bid to deter would-be thieves.
Avon and Somerset Police said they had not been informed of the incident. SeaQuarium staff said they had not informed police because no harm was caused to the rays.