The marine yabby or ghost nipper Callianassa australiensis is an intertidal burrow-dwelling shrimp found in estuaries along the eastern Australian coast. The marine yabby pumped water through its burrow at high tide by beating its abdominal appendages or swimmerets. Despite this activity, the burrow environment occupied by C. australiensis was relatively hypoxic, though the behaviour of the shrimp and the structure of the burrows enabled some parts of the burrow to be better ventilated than other parts. The water table remained high enough to allow the shrimps to maintain burrow ventilation for a considerable period after the tide had retreated, but ventilation activity declined gradually at low tide. Oxygen levels in the upper (and warmer) parts of the burrows were not dramatically depressed by emersion (13.58±4.72 kPa after 3h) . On the basis of these results, experiments were planned to determine the species' tolerance of hypoxia and to establish whether this shrimp could respire aerobically at low tide. This shrimp showed no significant change in swimmeret rate in response to oxygen tensions below its critical point (Pc) until it became anoxic and it differed from some other callianassid shrimps in not showing swimmeret hyperventilation upon exposure to rising oxygen levels after a period without oxygen. This result does not support the hypothesis that burrow ventilation by callianassid shrimps is a simple oxygen-mediated reflex. The oxygen uptake rate of this species (about 25 / µmo102 kg-1 min-1) was quite low for a shrimp of its size, (3.19g wet weight), and the shrimps were unable to sustain this oxygen uptake rate below an oxygen partial pressure of about 5kPa. Monitoring heart and gill bailer rate of C. australiensis in declining oxygen tension in a sealed chamber showed the same type of responses that have been reported from other decapods that are strong oxygen uptake regulators. The animal displayed compensatory scaphognathite hyperventilation and regulation of heart rate down to an oxygen tension of 1-3kPa, that is, slightly below the critical point of oxygen uptake (5kPa). Heart and gill bailer activity continued, albeit at a reduced rate, during environmental anoxia. Heart and bailer activity was not impaired when C. australiensis experienced acute exposure to oxygen tensions below 1.5kPa for 1h.Lactate levels in blood samples removed from shrimps in the field were low, and on that occasion decreased during low tide. However, replicate measurements of lactate concentration in whole shrimp showed a slight but significant rise after the tide went out. There may be a relationship between the rise in lactate concentration in whole shrimps at low tide and the arrest of surface activity occurring at this time. Two aspects of this complex problem are that the shrimps may experience hypoxia because they retreat from the warm surface water or that they become less active at the surface as a means of controlling the degree of anaerobiosis that they experience at low tide.