Earthworms, Eisenia foetida (Savigny), were maintained for 48 h in a nitrite-enriched artificial soil. Evidence suggests that animal cata- lase plays an important role in limiting the bio- accumulation of this toxicant (a widespread compound in the natural habitat of Eisenia). Nitrite oxidation is regarded as a biochemical mechanism: (1) coadapted with other aspects of functional ecology of earthworms; and (2) capable of enriching soils with nitrate, a nitrogen form available for plant metabolism.