Starting with the second day after fertilization, largemouth bass embryos and larvae became increasingly sensitive to oxygen deficiency until the sixth day of life. On this day few survived exposure to 1 mg O2/l for 3 h at 20C; many were damaged at 2 mg/l, but not at 2.5. Death was by asphyxiation or by starvation resulting from an apparent inability to close the lower jaw. On the seventh day the larvae became more resistant again, possibly because opercular movement began on that day. At 25C the effects of reduced oxygen concentration were intensified, and even a concentration of 2.5 mg/l became lethal in 3 h. At 3 mg O2/l and 20C the normally quiescent yolk-sac larvae became very active and swam vertically to at least 5 or 6 cm above the substrate; 4 and 5 mg/l had this effect at 23-24C. This behavior could result in losses by predation and displacement from the nest.