1. Survival and behaviour studies were made on ammocoetes subjected to water of various tensions of oxygen and carbon dioxide. 2. Ammocoetes can tolerate, for at least 4 days, oxygen tensions as low as 7-10 mmHg at 5 °C, 12-16 mmHg at 15.5 °C and between 13-16 and 19-21 mmHg at 22.5 °C. A limited ability to acclimate to low oxygen tensions was found in some animals. 3. A characteristic emergence behaviour is evoked by low partial pressures of oxygen that approach the lethal level and by high concentrations of carbon dioxide. 4. Ammocoetes respond to low oxygen and high carbon dioxide by an increase in the rate and amplitude of beating of the branchial basket. This increase is maintained in animals able to survive at low oxygen tensions. 5. A high affinity of the blood for oxygen is evident from oxygen equilibrium curves determined on erythrocytes suspension. There was an insignificant Bohr effect at 15.5 °C in the pH range 7.68-6.70, although a change occurred in the n value. 6. Haemoglobin concentration, haematocrit and oxygen equilibria suggest that the characteristics of the blood contribute significantly to the ability of ammocoetes to survive in low oxygen conditions.