Lactate dehydrogenase (LDH, EC 1.1.1.27) isozymes were studied from the white muscle and hearts of four species of Amazon fish, Osteoglossum bicirrhosum, an obligate water breather, and the closely related air breather, Arapaima gigas; Hoplias malabaricus, an obligate water breather, and Hoplerythrinus unitaeniatus, a facultative air breather. The LDH pattern was similar in closely related species but could not be correlated with changes in respiratory habits. All the isozymes from both heart and white muscle showed low substrate inhibition by pyruvate. The LDH from Osteoglossum skeletal white muscle showed sigmoidal kinetics with respect to pyruvate, rendered hyperbolic by the addition of 20 mM phosphocreatine. The kinetic properties appear to facilitate a dual function for white muscle in the absence of red muscle in the myotome. Phosphocreatine inhibition of LDH was observed in all the tissues studied and is suggested to be a more general phenomenon than has been previously supposed.