1. The respiratory responses of two mud-shrimps, Callianassa californiensis and Upogebia pugettensis (Thalassinidea), from Yaquina Bay, Newport, Oregon, were measured. 2. Both species are metabolic regulators, showing oxygen-independent respiration above the critical oxygen tension for C. californiensis of 10-20 mm Hg (6.- 12.5% air-saturation) and for U. pugettensis of 45-50 mm Hg (28-31% air-saturation). 3. Within the independent range of respiration, C. californiensis has a mean metabolic rate of 0.029 ml O2 x g wet wt-1 x hr-1, which is significantly lower than that of U. pugettensis (0.059 ml O2 x g wet wt-1 x hr-1). 4. Heart rates of C. californiensis subjected to diminishing oxygen tensions show a regulatory pattern similar to the metabolic rate, with bradycardia occurring at ca. 27 mm Hg. 5. Both species are tolerant to anoxia. C. californiensis survives approximately 5.7 days and U. pugettensis 3.3 days under such conditions. 6. Preliminary data suggest that postmolt U. pugettensis do not regulate and therefore are oxygen-dependent throughout the range tested. 7. The mean concentration of oxygen in water obtained from exposed U. pugettensis burrows is 0.58 ml O2/1, well above that of interstitial water. 8. C. californiensis, in contrast to U. pugettensis, does not construct firm burrows and is probably directly exposed to hypoxic interstitial waters. 9. Both species have respiratory adaptations for survival in a hypoxic environment. Quantitative differences in the metabolic requirements of the two species reflect the availability of oxygen in their respective niches within the mudflat biotope.