Soils under rape, barley and fallow were monitored for S immobilization, microbial biomass and arylsulphatase (ARS) activity. These soils were regularly collected in the field every fortnight from mid-April to the end of June 2001. Then, they were incubated for 1 week at 25_C with carrier-free 35S (Na2 35SO4). The results showed significant correlation of both immobilized 35S (r=0.94, P<0.001) and ARS (r=0.79, P<0.001) with microbial biomass-35S (MB-35S) but not with microbial biomass-C (MB-C). Therefore, only a fraction of MB-C was involved in the immobilization of 35S and hence the release of ARS. In addition, positive correlations appeared between hot water-soluble C (HW-C), 35S immobilization, MB-35S and ARS, indicating the pivotal role of HW-C in regulating S immobilization and turnover. The mean percentage values of immobilized 35S, MB-35S and ARS were higher in soil from the fallow, followed by soils from barley and rape. However, under culture, the mean values of ARS per unit of MB-35S showed the decreasing order: soil from rape (0.58)>soil from barley (0.46)>soil from fallow (0.34). It is concluded that microorganisms under rape were most efficient in producing ARS, a strategy used to cope with the SO4 2-- S limiting conditions