Description:
<jats:title>Abstract</jats:title><jats:p>Widespread anticoagulant resistance was discovered in populations of the brown rat (<jats:italic>Rattus norvegicus</jats:italic> Berk.) in an area of roughly 8000 km<jats:sup>2</jats:sup> in north‐west Germany. Resistance testing was performed by feeding tests and/or by measuring blood clotting response after intraperitoneal injection of a sublethal testing solution. A hierarchical resistance system was found with warfarin resistance at the base followed by bromadiolone/coumatetralyl resistance to difenacoum resistance at the top. Warfarin resistance was spread over the whole area with reduced incidence towards the edges. Difenacoum resistance represented the highest level found so far and was restricted to the inner zone of the resistance area where it occurred with a frequency of 6% of all individuals tested.</jats:p><jats:p>Breeding experiments with bromadiolone‐resistant rats showed that expression of the bromadiolone resistance gene differed in the two sexes, suggesting additional sex‐linked modifying effects to the resistance gene. Control strategy within the resistance area with respect to prevention of further selection for resistance is discussed.</jats:p>