Sublethal levels of suspended conifer groundwood impaired oxygen uptake and swimming ability of fathead minnows (Pimephales promelas). In fiber-free water active metabolism at 15, 18, and 21 C was 11.7, 13.0, and 14.9 mg O2/kg of fish per min, respectively, when oxygen was not limiting. Fiber suspensions of 100 to 800 ppm were tested, and at 21 C for each doubling in fiber concentration, active metabolism was reduced by about 1.3 mg/kg per min and the decrease was about the same at all dissolved oxygen (D. O.) levels. At 15 C, for each doubling in fiber concentration, active metabolism was reduced by 1.0 mg/kg per min at low D. O. levels (2.5 ppm) and by 0.2 mg/kg per min at high D. O. levels (9.0 ppm). Swimming endurance was reduced by suspended fiber with the effect being greatest at low D. O. levels. Hematocrit rose by approximately 2 for each doubling in concentration of suspended fiber and rose by 1.4 for each 1 ppm decrease in D. O. Gill-cleaning reflexes increased markedly with increase in fiber concentration and this activity was considered to raise the energy requirements for maintenance. Lowered active metabolism and lessened swimming endurance are considered to indicate stress conditions and to decrease survival and production of fish in natural habitats.