One of the basic requirements of fumigation methods (chlorofom fumigation methods consists in the fact that microorganisms put to death by chloroform vapours is complete, is principle. The present study describes the test of fumigation efficiency on the soil microflora depending on its activity. Tests were carried out on field sail (great Soil group - Cambisol, pH(H2O) = 5.9, C(ox) = 1.2 %), activity of soil microflora was increased either by moisturing. or glucose solution added (0.6 mg per 1 g of dry soil). Direct counts (fluorescence microscopy) of bacteria were measured before and after fumigation in the soil along with representation of ammonizing bacteria sporulating and unsporulating, oligotrophic, oligonitrophic and N-fixing bacteria. actinomycetes and micromycetes (numbers of colonics grown on various apr media). Moreover, the effect of efficiency of fumigation has been studied as affected the increase in respiration after fumigation. There were more microorganisms survived in the fumigated soil at low moisture (2 % of MVK) than in the soil fumigated at higher moisture content (55 % of MVK as shown in Tab. I). In comparison with the soil unfumigated, CO2 formation by fumigated soil was higher, though the increase was more marked in the soil fumigated at the moisture content 55 % of MVK than at the low moisture content 2 % of MVK (Fig, 1). Amount of microbial biomass 158 mug C.g-1 was calculated in the same soil sample (fumigation at the moisture content 2 % of MVK) and 261 mug C.g-1 (fumigation at 55 % of MVK). Moreover, the soil was fumigated before glucose added and then gradually 2 to 192 hours after glucose added. With longer time of incubation of samples containing glucose, i.e. with increasing microbial activity, percentage of bacteria surviving fumigation was failing and on the contrary, with falling activity after withdrawing of glucose, the amount of survived microorganism was rising (Fig. 2). It is obvious on the basis of detail data referred to representation of selected physiological groups of microorganisms that the sensitivity of all studied groups to fumigation is dependent on their activity (Tab. II). Generally, due to the fumigation, the counts of actinomycetes, ammonizing bacteria and oligotrophic bacteria, the least - the counts of sporulating bacteria decreased. The efficiency of fumigation was also studied in the soil samples collected under different temperatures and analyzed immediatelly after sampling. In the soil sampled at low temperatures, i.e. in the time of low activity of microorganisms, the lower efficiency of fumigation was determined (Tab. III). It follows from the given results that a low level of metabolic activity of microorganisms affects negatively the efficiency of fumigation by chloroform vapours. This is the source of errors made during using the fumigation methods to determinate microbial biomass.