AbstractThe tolerance of the mudskipper,Boleophthalmus boddaerti, to cyanide is higher than those of other fishes reported elsewhere. The 24 h, 48 h, and 96 h LC50forB. boddaertiwere 0.37 mg HCN/liter, 0.34 mg HCN/liter, and 0.29 mg HCN/liter, respectively. The tolerance ofB. boddaertito cyanide does not involve a reduction of metabolic rate or an enhanced anaerobic metabolism, or a cyanide‐insensitive respiratory pathway. Instead, it is a result of the presence of a surplus of cytochrome oxidase and inducible cyanide‐detoxifying mechanisms in this mudskipper. Although cytochrome oxidase activities in the various tissues of the fish exposed to 0.23 mg HCN/liter were inhibited by more than 50%, no reduction in the O2consumption rate was observed.B. boddaerticould detoxify cyanide to thiocyanate. The activity of rhodanese in the liver extract of fish exposed to cyanide increased significantly compared to that of the control. The activities of cysteine amino‐transferase (CAT) and 3‐mercaptopyruvate sulfurtransferase (MPST) in the extracts from the liver and the muscle ofB. boddaertiexposed to cyanide also increased significantly compared to those of the control fish. Glutamate formed in the reaction catalyzed by CAT could undergo transamination leading to the accumulation of aspartate in the muscle of the fish exposed to