The hungry frogs are about the same size as a pencil case, and with the amount of frogspawn they produce, there could be thousands of them soon.
The experts think that animals that are supposed to be living in Britain could be eaten out of existence by the newcomers.
They got here by accident
It's thought the bullfrogs got into Britain by accident, as tadpoles in bags of imported pond plants or fish.
Anybody who finds a North American bullfrog can contact Froglife charity on 01986 873733.