The fumigation/respiration technique was used to estimate the size of the soil microbial biomass. Sieving decreased the biomass in winter but increased it in summer; we suggest that this was a consequence of the different substrates available and the different microbial populations during the year. The flush in respiration following fumigation correlated significantly with the CO2-C produced 10 days after fumigation (X), so that in the soils studied by us the biomass (B) can be calculated from Bk=0.673X-3.53, wherek is the fraction of fumigated organisms mineralized to CO2, thus avoiding the need to measure CO2 production from unfumigated cores.