Abstract 1. 1. The horseshoe crab, Limulus polyphemus, can survive 36–60 hr at 22°C under virtually anoxic conditions. 2. 2. Phosphoeonolpyruvate carboxykinase (PEPCK) activity increased 2–3 times during the period of facultative anaerobiosis in both hepatopancreas and muscle tissue. 3. 3. Heart rates decreased to 25 per cent of the basal rate under these conditions, implying partial circulatory shutdown. 4. 4. No difference in survival was noted between organism exposed to acute anoxia or over a 24-hr acclimation period. 5. 5. Unlike the situation in the oyster or parasitic helminths (also capable of facultative anaerobiosis), PEPCK appears to be a mitochondrial enzyme and exhibits only slight cytosol activity. Evidence of facultative anaerobiosis in Limulus suggests an ability to adapt to oxygen-limited environments for short lengths of time.