Recordings from burrows of the filter feeding upogebiid mud-shrimps U stellata and U deltaura, constructed in laboratory aquaria, showed a gradient of oxygen tension with depth. The burrow water at the deeper parts of the burrow was often severely hypoxic. The water pH also decreased through the burrow, reflecting an increase in the PCO2 with depth. Long term changes in the burrow water PO2 are dependent upon a number of factors of which burrow irrigation is the most important. Irrigation not only provides a feeding current but also serves to replenish the oxygen-depleted burrow water. The degree of exchange of burrow water is dependent both on the amount of time spent irrigating and the rate of irrigation. Large temporal fluxes in oxygen tension occur within the burrow. These fluxes in PO2 are dependent upon the irrigatory behaviour, respiration of the occupant and of the meiofauna and microflora associated with the burrow wall and upon the diffusion of oxygen through the burrow system.