Fish that glow green when put in polluted water have been created in a British lab.
It's hoped the luminous zebra fish will help improve the cleanliness of our rivers and help identify dangerous chemicals that get in to our water supply.
The fish are designed so that the parts of their bodies harmed by pollutants light up.
The advance was made at Exeter University.
Chemicals from paints and many plastics often find their way in to our rivers, causing many creatures problems breeding.
Certain pollutants can even harm humans, so this development is seen as very important.
Professor Charles Tyler who led the study said: "This zebrafish gives us a better view than ever before of the potential effects of these chemicals on the body. It is very exciting."