The efficacy of the anticoccidial drug diclazuril against field isolates of Eimeria spp. collected from broiler farms in four different sites of the South and Southeastern regions of Brazil were investigated. The effect of the drug was measured by an assessment of weight gain, lesion score and oocyst production. Two reference laboratory strains not previously exposed to diclazuril were sensitive to the drug. Two field isolates were judged to be sensitive to diclazuril, four were partly resistant and six were resistant to the drug. The response of the isolates to diclazuril varied depending upon the extent of exposure and type of drug programme in use at the farms.232305311