Lots of hedgehogs are dying in Britain and researchers think that rat poison could be the reason for their problems.
It's thought that people are putting poison down to kill rats, which is harming hedgehogs by accident.
Scientists from the University of Bristol studied the bodies of 120 dead hedgehogs and they found poison in 80 of them.
Animal charity the RSPCA wants people to think more carefully about the methods they use to get rid of rats.
Dr Claire Dowding, who carried out the research, says it's difficult to know exactly how the hedgehogs come into contact with the dangerous chemicals.
She says they could be eating the poison themselves, or it could be getting their bodies from the animals they munch on.
The hedgehogs could be snacking on rats that have been poisoned, or may get the poison from their fave food - snails and slugs.
The slimy creatures eat the poison too, but aren't harmed by it themselves. Even so it can stay in their bodies until the hedgehogs catch up with them.