Diclazuril is an anticoccidial that inhibits tachyzoite production of the RH strain of Toxoplasma gondii by > 97% at a concentration of 0.005 micrograms/ml. The effects of 1.0 microgram/ml diclazuril on the development of 3 strains (RH and 2 tissue cyst formers, GT-1, and WTD-3) of T. gondii in vitro was examined using transmission electron microscopy. The effects of diclazuril were not noted until 2 days after treatment. Treatment with diclazuril interfered with endodyogeny and resulted in the production of multinucleate (> 2 nuclei) meront stages. Up to 12 nuclei were observed in some meronts. Tachyzoites attempted to bud from the surfaces of these stages but division was apparently blocked by the action of diclazuril. The parasitophorous vacuole (PV) enclosing developing stages became hypertrophic. As the meront stages degenerated the PV became filled with membranous material and the cytoplasmic contents of lysed stages. Formation of tissue cysts by the GT-1 and WTD-3 strains of T. gondii was not prevented by treatment with diclazuril. A mutant of the RH strain that was resistant to 1.0 microgram/ml diclazuril was selected by progressive culture in permissive levels of the agent. This DicR-1 mutant produced significantly fewer tachyzoites (P < 0.05) in vitro than its parent strain. Resistance to diclazuril appears to be stable because reversion to sensitivity to the agent did not occur after 50 passages in the absence of drug pressure. The DicR-1 mutant was somewhat less pathogenic for mice (70% mortality) than its parent RH strain (100% mortality) and persisted as tissue cysts in the brains of mice.(ABSTRACT TRUNCATED AT 250 WORDS)