A b t s r a c t. The aim of this work was to fiut relations between the number of selected groups of soil micro-organisms, water stability of soil aggregates, soil porosity and the organic matter content on the basis of field experiment. Two types of soil were used simultaneously for the research: chemozemic rendzina and brown soil. Rotation system was used in plant cultivation. Chemical, physical and microbiological analyses were carried out on the control object and on objects with organic fertilizers and mineral fertilizers N, P, K. Differences in the sum of fractions of organic C, chiefly in the easily oxidizing fractions in experimental fields on rendzina, in comparison to those on brown soil, can be the result of the predominance of zymogenous bacteria over autochtonous bacteria in the rendzina. Easily oxidizable fractions of C contained in the organic substance of particular fields can be utilized by a high ratio of zymogenous bacteria in the soil samples. It should be noted that characteristic differences in the number of bac­teria have been observed in the two types of soils examined. The number of autochtonous microorganisms in brown soil was twice as much as in rendzina. Zymogenous microorganisms constitute more than 90 % of the total number of microorganisms in rendzina. Underthe cultivation of red clover an inerease of water stability of soil ag­gregates was stated. From the porosimetric analyses no changes under different cultivation of two soils were observed. The ratio of microorganisms was connected with the type of soil and the system of cultivation.