The nitrogenase activity, root nodule biomass, and rates of nitrogen (N) fixation were measured in 25-year-old pure north- and south-facing Robinia pseudoacacia stands in an urban forest of Seoul (Kkachisan Mountain) in central Korea. The nitrogenase activity was estimated using an acetylene reduction (AR) assay, which showed an increasing trend during the early growing season, with sustained high rates from June through to September with a decrease thereafter. July had the highest nitrogenase activity rate (micromoles $C_2H_4$ per gram dry nodule per hour), averaging 95.8 and 115.1 for the north- and south-facing stands, respectively. The maximum root nodule biomass (kilograms per hectare) was 45.7 and 9.1 for the north- and south-facing stands in July, respectively. The AR rate appeared to be strongly correlated to the soil temperature ($r^2$=0.68, P<0.001) and soil pH ($r^2$=0.59, P<0.001) while root nodule biomass was correlated to the soil temperature ($r^2$=0.36, P<0.01) and water content ($r^2$=0.35, P<0.05). The soil temperature showed clear differences between seasons, while there was a significant difference in soil pH, organic matter, total N concentrations, and available phosphorus between the north- and south-facing stands. The $N_2$ fixation rates during the growing season varied from 0.1 to $37.5;kg;N;ha^{-1}month^{-1}$ depending on the sampling location and time. The annual $N_2$ fixation rate (kg N per hectare per year) was 112.3 and 23.2 for the north- and south-facing stands, respectively. The differences in $N_2$ fixation rate between the two stands were due mainly to the differences in total nodule biomass.