Sulfide is toxic for most aerobic organisms and inhibits the key enzyme of aerobic respiration, cytochrome c oxidase, at low μM concentrations. Tubificoides benedii is a ubiquitous marine oligochaete that regularly occurs in polluted, poorly oxidized coastal muds with high sulfide concentrations. The questions posed in this study were: Can T. benedii maintain aerobic pathways despite the occurrence in its habitat of sulfide concentrations that are toxic for many marine invertebrates? What influence does the ambient oxygen concentration have on the sulfide effects? To answer these questions animals were incubated under flow-through conditions in various sulfide and oxygen concentrations. As an indicator of an anaerobic metabolism succinate was measured, a metabolite which accumulates in high concentrations in T. benedii under hypoxic conditions. The results show that under normoxic conditions, T. benedii can maintain a completely aerobic metabolism at sulfide concentrations up to 175 μM. At 300 μM sulfide, anaerobic pathways are utilized to only a small extent and even at sulfide concentrations as high as 450 μM and 600 μM a partially aerobic metabolism is sustained. However, if ambient oxygen concentrations are decreased, much lower sulfide concentrations cause the animal to utilize anaerobic pathways. At oxygen concentrations as low as 20 μM (= 1.5 kPa O2 or 7% air saturation), where the worms can maintain a fully aerobic metabolism in the absence of sulfide, minimal concentrations of sulfide (40 μM) are sufficient to give rise to an anaerobic metabolism. © 1994 Wiley-Liss, Inc.