Thiabendazole, 2-(4′-thiazolyl) benzimidazole (TBZ) inhibited the growth of Penicillium atrovenetum at 8 to 10 μg/ml. Oxygen consumption with exogenous glucose was inhibited at 20 μg/ml, but endogenous respiration required more than 100 μg/ml. TBZ inhibited completely the following systems of isolated heart or fungus mitochondria: reduced nicotinamide adenine dinucleotide oxidase, succinic oxidase, reduced nicotinamide adenine dinucleotide-cytochrome c reductase, and succinic-cytochrome c reductase at concentrations of 10, 167, 10, and 0.5 μg/ml, respectively. Cytochrome c oxidase was not inhibited. Antimycin A and sodium azide caused the usual inhibition patterns for both fungus and heart terminal electron transport systems. In the presence of antimycin, the fungicide inhibited completely succinate-dichloro-phenolindophenol reductase and succinate-2, 2-di-p-nitrophenyl-(3, 3-dimethoxy-4, 4-biphenylene-5, 5-diphenylditetrazolium)-reductase at 2 and 4 μg of TBZ per ml, respectively. Coenzyme Q reductase required 15 μg/ml. TBZ reduced the uptake by P. atrovenetum of glucose and amino acids and decreased the synthesis of various cell components. At 120 μg/ml, the incorporation of labeled carbon from amino acids-U-14C was decreased: lipid, 73%; nucleic acids, 80%; protein, 80%; and a residual fraction, 89%. TBZ did not inhibit peptide synthesis in a cell-free protein-synthesizing system from Rhizoctonia solani. Probably the primary site of inhibition is the terminal electron transport system and other effects are secondary.