The mitochondrial adenosine triphosphatase of the kinetoplastid protozoon, Crithidia fasciculata, is inhibited by oligomycin, venturicidin, triethyltin sulphate, N,N'-dicyclohexylcarbodiimide, leucinostatin, Dio-9 and quercetin, but not spegazzinine or by compounds which interact with the beta-subunit of mitochondrial F1-ATPase (efrapeptin, aurovertin, citreoviridin or 4-chloro-7-nitrobenzofurazan). These results suggest that the F1 portion of the crithidial enzyme has an unusual type of beta-subunit. Further evidence for the atypical nature of this enzyme is provided by the observation that F1-inhibitor proteins from Acanthamoeba castellanii or bovine heart mitochondria do not inhibit the C. fasciculata enzyme activity.