Microbial biomass (C(mic)), microbial activity (SRA) and soil respiration (SR) were studied in a secondary successional row represented by an intensively cultivated field, by 2 - 10 years uncultivated fields (fallow and meadow, respectively) and by an oak forest. Regular monthly observations were made over 3 years. C(mic) and SR were lowest in the fallow and field soils and increased according to soil organic carbon content from 0.34 to 0.55 mg C . g-1 and from 0.019 to 0.034 mg CO2 - C . g-1 . d-1, respectively. On the contrary, SRA was lowest in the meadow soil (0.068 Mg CO2 -C . Mg C-1 . d-1) but reached the highest values in the field (0.113 mg CO2 - C . Mg C-1 . d-1). Temporal fluctuations of C(mic) and SRA decreased in the order field - fallow - meadow and forest. The C(mic) and SRA variations, turnover rate of C(mic), and the portion of C(mic) in the soil organic carbon are discussed.