The rate of feeding of E. brevis, which contains haemoglobin in the pharynx, was measured over a wide range of oxygen concentrations, and compared with similar results for a second species, E. communis, which lacks haemoglobin in its pharynx. The nematodes were added to a mixture of mud and amaranth, in varying oxygen regimes, and the dye content of the disrupted individuals was measured two days later by a spectrophotometric technique. The feeding rate of E. brevis was less affected than that of E. communis by partial pressures of oxygen between 8 and 40 mmHg, but both species ingested some mud in almost anaerobic conditions. Comparison of the vertical distribution of E. brevis with the redox potential of its mud habitat showed that the animal is only occasionally found in anoxic mud. It is suggested from these results, and from previous work, that the facilitation of oxygen diffusion by the haemoglobin in the pharynx of E. brevis helps this nematode to feed in the low and fluctuating oxygen regime of its habitat.