German scientists are on the lookout for "super rats" resistant to rat poison.
Brown rats resistant to traditional rat poison have turned up in several parts of Europe and in North America, Der Spiegel reported.
Some are also immune to newer poisons, the newspaper said.
"We've discovered the mutants in some areas of Germany as well," Hans-Joachim Pelz of Germany's Biological Research Center for Agriculture and Forestry told Der Spiegel.
Scientists are examining DNA from rats in Hamburg and Lower Saxony to see if the "super rat" gene is spreading through Germany, Der Spiegel reported.
Berlin exterminator Reinhard Gajek said he hopes they don't find any mutant rats.
"You're always afraid the stuff will stop working at some point," he told der Spiegel.
Copyright 2007 by United Press International