Summary
Silurideos species from the families: Doradidae, Aucheniptaridae, Pimelodidae, Ageneiosidae, Callicthyidae and Loricariidae, were analysed by polyacrylamide gel electrophoresis. The results allow us to suggest that very active fishes which live in different habitat, present hemoglobin with exclusively anodic mobility. On the other hand, fishes adapted to more severe condition such as those of high hypoxy including fishes with aerobic respiration show hemoglobins with electrophoretic patern highly heterogeneous presenting anodic and cathodic components. A mechanism is proposed to explain the heterogenity as well as the electrical mobility of the hemoglobin components related to the habitat extremely variable of the Amazon River.