Pen field trials were conducted to assess the performance of the acute rodenticide pyriminil against the house mouse (Mus musculus L.). Four types of poison treatment were carried out using penned family groups of warfarin-resistant mice supplied with alternative plain foods. In each treatment pyriminil was included at 2% in a wholemeal flour/pinhead oatmeal/corn oil bait. Mortality was highest (46/54; 85.2%) when poison bait was offered for 4 days following 3 days of pre-baiting. The same pre-baiting and poisoning technique was adopted in five field trials carried out against mice infesting farm buildings. The efficacy of each poison treatment was estimated from the results of pre- and post-treatment census baitings; treatment success ranged between 53.7% and 96.7%, mean 80.5%. It is concluded that pyriminil treatments are best carried out after a period of pre-baiting and that when pyriminil is used in this manner it is about as effective as zinc phosphide for the control of mice.