Efficiency of warfarin anticoagulant was studied against different populations of the black rat, Rattus :anus, caught from 4 districts at Sharkia govemorate (Belbies, Kafr-Sakr, Menia El-Khamh and Hehya dis­tricts) using biological method (lethal feeding periods "LFP") and bio­chemical technique (prothrombin time "P.T" determination). The results showed that the highest values of LFP50 and LBP9 9 were recorded with rats trapped from Hehya (3.4 and 9.9 days), while the lowest values were in case of rats trapped from Belbies (2.6 and 6.9 days). Data ob­tained from biochemical test were in the same trend whereas effect of warfarin (5mg/kg) on P.T. values was obviously noticed in animals caught from Belbies, while the lowest effect was observed on the ani­mals of Hehya. Results indicated that rats trapped from Hehya showed a high tolerance to warfarin, while those caught from Belbies were relative­ly susceptible followed by those from Kafr-Sakr and Menia El-Khamh dis­-tricts.