The effectiveness of calciferol (vitamin D(2)) against Rattus norvegicus was investigated in field trials on twenty-three farms with rat infestations partly resistant to warfarin. At 0.01% combined with warfarin at 0.025% no rodenticidal effect of the calciferol was discernible. At 0.025% with warfarin at the same concentration, results were better, but not appreciably better than is often obtained with warfarin alone against resistant rat populations. When the concentration of calciferol was stepped up to 0.1%, four out of five treatments in which the poison was applied directly gave complete control. The fifth may have partly failed because of poison shyness caused by under-baiting. Five out of six more treatments done after ;pre-baiting' were also successful. The sixth failed for reasons unconnected with the choice of poison.Six further infestations that were not responding adequately to warfarin treatments were quickly controlled when, in three instances, calciferol at 0.1% was used instead and, in three more, it was used together with warfarin. It is concluded that calciferol at 0.1% is an effective poison against R. norvegicus either combined with warfarin or not, but that because at 0.1% its effect is subacute rather than chronic, there may be a case in some environments for using it only after pre-baiting.