1. The importance of Hb for certain aspects of goldfish respiration has been tested by converting it to COHb.

2. Goldfish exposed to 80% CO and 20% O2 survived over 24 hr. at 30° C., and indefinitely longer at lower temperatures.

3. Routine activity of goldfish over most, if not all, of their thermal range does not depend upon Hb.

4. Removal of dissolved O2 from water under asphyxial conditions does not depend upon Hb.

5. While the physiological basis of CO2 tolerance curves remains unresolved, it can be said that they do not indicate an effect of CO2 upon Hb.

6. Apparent changes in sensitivity to CO2 that have been ascribed to changes in thermal environment are probably artefacts arising from variation in the solubility of CO2.

This study was carried out under tenure of scholarships from the National Research Council of Canada, and with assistance during one year from Department of Veterans Affairs, Canada