Mitochondrial adenosine triphosphatase (ATPase) of the ciliate protozoon Tetrahymena pyriformis ST is completely inhibited by antiserum prepared against F1-ATPase purified from Schizosaccharomyces pombe, and by naturally occurring inhibitor proteins from this yeast and from bovine heart mitochondria. An ATPase inhibitor protein is also present in extracts of T. pyriformis. Mitochondrial ATPase of T. pyriformis is only partially inhibited by the F0-ATPase inhibitors N,N'-dicyclohexylcarbodiimide, oligomycin, leucinostatin, triethyltin sulphate and venturicidin, and (at high titres) by the F1-ATPase inhibitors Dio-9, efrapeptin, 4-chloro-7-nitrobenzofurazan and spegazzinine. Aurovertin, citreoviridin and quercetin were not inhibitory. Resistance to inhibitors distinguishes this mitochondrial ATPase from all those previously examined.