Roxarsone was tested for anticoccidial efficacy against several individual and mixed field isolates of Eimeria tenella, E. acervulina, E. maxima, and E. brunetti in laboratory tests, and for its contribution to the anticoccidial efficacy of commonly-used anticoccidial products. Roxarsone displayed moderate to good activity against E. tenella and E. brunetti at 45.4 g/ton, but only marginal activity against other species. When used in combination with ionophores, there was considerable improvement in control of field isolates that were known to be resistant to the ionophores alone. Overall, there was good evidence that improvement in performance under field conditions resulted from the anticoccidial activity of roxarsone rather than its growth promotion characteristics.