Laboratory feeding tests were conducted to determine the efficacy of 0.025% warfarin against two separate populations ofRattus argentiventer(Robinson and Kloss) in Peninsular Malaysia. No-choice feeding periods of 2, 4, 6, 8, 10 and 12 days gave mortalities of 27.5, 57.5, 82.5, 87.5, 97.5 and 100% respectively for the pooled data. No significant heterogeneity was found between the sexes of the populations but there was marked individual variation in susceptibility to warfarin. A base-line for the susceptibility ofR. argentiventerto warfarin was established; the upper 95% confidence limit calculated for the LFP (Lethal Feeding Period) 98 indicated that survival following an 18-day feeding period on 0.025% warfarin bait is a suitable test for the detection of warfarin resistance. It is concluded that warfarin is appropriate for use againstR. argentiventerinfesting rice fields and that poisoned bait would be best applied when alternative foods are most limited.