Many aquatic habitats are characterized by variable concentrations of dissolved oxygen (DO), and fish that occur in these habitats respond to changes in DO through behavioral, physiological, and biochemical adjustments. The goal of the present study was to measure the effects of an ecologically relevant range of DO treatments, from severe hypoxia to moderate hyperoxia, on the maximal activities of nine glycolytic enzymes during chronic exposure of the mummichog, Fundulus heteroclitus. Over the 28 days of exposure period, specific activity was significantly affected by DO for three enzymes in liver and one enzyme in white skeletal muscle, although at specific times of exposure three other muscle enzymes were affected by DO. In general, exposure of fish to severe hypoxia led to higher specific activities in liver, but lower specific activities in skeletal muscle. Exposure to hyperoxia did not elicit changes in enzyme specific activities in either tissue. Surprisingly, exposure duration had strong effects on glycolytic enzyme specific activities in both liver and white skeletal muscle, with specific activities increasing with exposure duration regardless of DO treatment. The results demonstrate that the effects of DO on enzyme specific activities were restricted to a subset of the glycolytic enzymes in liver and white skeletal muscle of F. heteroclitus and that the directions of the changes were opposite in these two tissues. These observations suggest that the mechanisms resulting in these alterations are enzyme- and tissue specific, rather than applying uniformly to all enzymes within the glycolytic pathway.