When 1-cm diameter, waterlogged, sandy clay loam soil aggregates were amended with 2% (w/w) cellulose or 2% (w/w) wheat straw, significant C2H2-reduction activity developed whether the aggregates were incubated under aerobic or anaerobic gas phases. The only N2-fixing bacteria which proliferated were anaerobes or facultative anaerobes. Aerobic N2-fixing bacteria did not proliferate in any of the aggregates, even though a few were initially present in the soil. The anaerobic bacteria in aerobically incubated, cellulose-amended aggregates proliferated more slowly than those in the anaerobically incubated aggregates, but eventually produced a higher maximum rate of C2H2-reduction. The activities of the N2-fixing bacteria in straw-amended aggregates were similar under both aerobic and anaerobic gas phases. High amendment levels (20% w/w) altered the soil structure and the requisite anaerobic microsites were not developed as rapidly under aerobic incubation conditions and so the onset of C2H2-reduction activity was delayed.