Smallmouth bass larvae became highly sensitive to oxygen deficiency on the second day after hatching and continued so to the 10th day. During this period they could not survive exposure to 1 mg O2/1 for 3 h at 20C, and many were killed within 1 h. At 2 mg O2/1 half the larvae survived 3 h at 20C; at 2.5 mg/1 most survived, and at 3.5 mg/1 all survived. Resistance to oxygen deficiency was regained by the 11th day, the majority of the larvae withstanding a 3-h exposure at 1 mg O2/1. At 25C the effects of low oxygen concentration were intensified.