Native fixed ammonium nitrogen and fixation of added ammonium nitrogen were studied in 24 Queensland cereal producing soils. The availabiiity of both forms of fixed ammonium nitrogen to wheat was investigated using a montmorillonitic soil under pot culture. Fixation and release of fertilizer ammonium nitrogen were followed in the soil by sequential sampling during crop growth. All soils contained native fixed ammonium nitrogen (6-107 p.p.m. nitrogen, mean 39 p.p.m. nitrogen). Fixation of added ammonium nitrogen was only significant in high clay montmorillonitic soils. Under moist conditions an average of 5% (200 p.p.m. nitrogen added) and 1% (4000 p.p.m. nitrogen added) was fixed. Fixation increased threeto ten-fold following air drying. Intensive cropping only released small but non-significant amounts of native fixed ammonium nitrogen. Of the 15% of added ammonium nitrogen which was fixed, only 50% was released by a single low density cropping causing less nitrogen uptake by wheat tops from ammonium compared with nitrate nitrogen sources. Intensive or successive cropping released almost all recently fixed ammonium nitrogen.