The fixed ammonium content and the NH₄⁺-fixing ability of the main types of paddy soils in Taihu lake region were determined and the availabilify of the fixed ammonium was studied under greenhouse conditions. It was found that both the fixed NH₄⁺ content and the NH₄⁺-fixing ability of soils varied with the parent materials, they decreased in the order:Yangtze River alluvium>loess-like material>Quaternary red clay. About 18% of the total N in the soil layer of 0-20 cm and 34% of that in the soil layer of 0-100 cm were in the form of fixed NH₄⁺-N.The availability of ehe "native" fixed ammonium was rather low,and varied considerabiy in diferent horizons, ranging from 0 to 13%, while that of the newly fixed NH₄⁺ was very high, berg nearly 100%. There was no evidence showing the effect of waterlogging on the availability of the fixed NH₄⁺. The significance of NH₄⁺-fixation in soil N,fertility is discussed, arid it is suggested that the values obtained by the submerged incubation method were not only lower than the real ones but also incomparable.-due to the presence of NH₄⁺-fixation and difference in NH₄⁺-fixing ability among various soils.