1. An apparatus is described for continuously measuring gill ventilation volumes in crabs.

2. Large Carcinus pump about 1 c.c./g./min. and consume oxygen at the rate of about 0.03 c.c./g./hr. whilst smaller specimens pump up to 1.5 c.c./g./min. and consume up to 0.1 c.c./g./hr. Freshly collected crabs show persistent tidal and 24 hr. rhythms of pumping activity and oxygen consumption.

3. In response to oxygen depletion Carcinus shows increased rates of gill ventilation and increased percentage utilization. Prolonged exposure to increased carbon dioxide results in a short-lived inhibition followed by over-compensation and then progressive inhibition of respiratory activity.

4. The results are discussed in relation to previous work on respiration in other decapod Crustacea.

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