Adenine arabinoside, which inhibits the synthesis of DNA by intracellular Toxoplasma gondii, is more inhibitory to the parasite than it is to cultured human fibroblast host cells. A single-step mutant of T. gondii that is 50-fold more resistant to adenine arabinoside was isolated after mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine. Cytosine arabinoside is notably more inhibitory to cultured human cells than it is to T. gondii. Thus in infected cells treated with cytosine arabinoside and labeled with 3H-deoxyuridine nearly all of the label is specifically incorporated into the intracellular T. gondii.