The gut of the earthworm constitutes a mobile anoxic microzone to which the microorganisms of aerated soils are subjected. During gut passage, the in situ factors of the earthworm gut, which include anoxia and high concentrations of organic substrates, appear to greatly stimulate a subset of ingested soil microorganisms, including denitrifying and fermentative bacteria. The selective stimulation of ingested soil microbes by the unique microconditions of the earthworm gut (a) results in the in vivo emission of denitrification-derived dinitrogen (N(2)) and the greenhouse gas nitrous oxide (N(2)O) by the earthworm, and (b) might affect the fitness, culturability, and diversity of certain members of soil microbial biomes. These observations illustrate the impact that soil macrofauna might have on terrestrial nitrogen cycle processes via their transient hosting of ingested prokaryotes.