The fate of Cs-137 derived from the Fukushima nuclear accident fallout and associated radiological hazards are largely dependent on its mobility in the surface soils of forest ecosystems. Thus, we quantified microbial and adsorptive retentions of Cs-137 in forest surface (0-3 cm) soils. The K2SO4 extraction process liberated 2.1%-12.8% of the total Cs-137 from the soils. Two soils with a higher content of clay-and silt-sized particles, organic carbon content, and cation exchange capacity showed higher Cs-137 extractability. Microbial biomass was observed in all of the soils. However, the Cs-137 extractability did not increase after destruction of the microbial biomass by chloroform fumigation, providing no evidence for microbial retention of the Fukushima-fallout Cs-137. The results indicate that uptake of Cs-137 by soil microorganisms is less important for retention of potentially mobile Cs-137 in the forest surface soils compared to ion-exchange adsorption on non-specific sites provided by abiotic components.