Three physiological variables, haematocrit, haemoglobin concentration and ventilation frequency, were measured to test how fathead minnows Pimephales promelas and small and large yellow perch Perca flavescens responded to three different dissolved oxygen concentrations. All fish were monitored continuously for any indications of stress in response to these manipulations. Within and between species, smaller individuals were the most tolerant of hypoxic environments. A species effect, however, did contribute to this observation, with fathead minnows being more tolerant of hypoxic environments than similar-sized yellow perch. In aquatic ecosystems where smaller fishes are more tolerant to hypoxia than their larger predators, hypoxic environments may have the potential to act as a refuge from predators.