One of the factors that change the physical and chemical properties of soil is the activity of micro and mesofauna, including the earthworms. The important influence of earthworms on aggregate and organic matter stabilization is often emphasized in literature. Earthworms are animals that swallow the soil and digest the residues of organic matter. Bacteria and algae cells, fungi mycellium cause that the chyme that goes through their alimentary system requires new properties. Hence, their excreta (casts) constitute a great environment for micro-organisms. The soil treated by earthworms is characterized by better aggregate structure, and has improved air-circulation thanks to the created micro and macro-pores. The aim of the research was to compare the number of bacteria and fungi in the soil treated by earthworms in the laboratory conditions under different soil water content. On the basis of the obtained data, evident differences in the number of bacteria and fungi, dependent on humidity in the soil treated by earthworms in laboratory conditions, were observed. The number of bacteria and fungi were calculated in soil, in casts and in burrows created by earthworms. The obtained data confirm the higher number of fungi in casts comparing to the soil. Incubation of samples at humidity of 23% by weight revealed a higher number of bacteria both in casts and in soil compared to humidity 17% by weight.