Decoquinate is an anticoccidial agent that inhibits respiration in the parasites mitochondrion. We examined human foreskin fibroblast cell cultures infected with the normally tissue cyst-less RH strain of Toxoplasma gondii and treated with decoquinate for evidence of tissue cyst induction and formation. Transmission electron microscopy observations demonstrated tissue cysts in decoquinate-treated cultures on days 3, 4, 5, and 6 after inoculation. Tissue cysts contained a tissue cyst wall that enclosed stages that resembled tachyzoites and stages that were structurally bradyzoites. Similar treatment of human foreskin fibroblast cells infected with tachyzoites of the TS-4 temperature-sensitive mutant of the RH strain did not result in production of tissue cysts.