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The eggs of Cyclops vernalis were gassed with varying quantities of oxygen and nitrogen, then allowed to hatch and the larvae to develop. A critical threshold of available dissolved oxygen was observed at 1.34 mg O2/l; below this level, hatching dropped to 15%. At 0.38 mg/l hatching did not occur. A similar threshold of survivorship (23%) was observed at 1.34 mg O2/l. Below this level, no organisms reached the first copepodid stage. Survival above 70% required 2.67 mg O2/l dissolved oxygen. Most naupliar mortality occurred at the initiation of ecdysis reflecting the stress imposed by oxygen crisis. A detailed description of the gassing procedure and apparatus is provided.