Vertisols originally carrying brigalow vegetation (Acacia harpophylla F. Muell. ex Benth.) and cultivated for cereal cropping for up to 45 years were examined for trends in available phosphorus (P) [sodium bicarbonate extractable P (bicarb. P) and dilute acid extractable P (acid P)], organic P, inorganic P, and total P. The soils (0-0 · 1 m depth) in their virgin state contained 60 mg/kg of bicarb. P, 168 mg/kg of acid P, 239 mg/kg of organic P, and 330 mg/kg of inorganic P. All fractions of soil P declined following first-order decay with the period of cereal cropping; the rates of bicarb. P and acid P (available P) decline were 0 · 047 and 0 · 08/year. The organic P, inorganic P, and total P declined more slowly than available P; the respective rates were 0 · 026, 0 · 019, and 0 · 021/year. The rates of loss of total P and inorganic P were much higher from the clay-size fraction than the silt-size or sand-size fraction, with the t½ value of inorganic P in the sand-size fraction being almost 20 times greater than any other fraction. On the other hand, organic P loss from the clay-size fraction was much less; presumably, clay provides physical protection to soil organic matter and hence to organic P from decomposition. On average, bicarb. P and acid P declined at the rate of 1 · 3±0 · 3 and 4 · 6±1 · 4 mg P/kg soil · year. The declines in organic P and inorganic P were 3 · 0±0.4 and 4 · 1±1 · 2 mg P/kg soil · year, with a loss in total P of 7 · 2±1 · 3 mg P/kg soil · year. There was no significant shift in the ratio organic P: total P (38±7%) with the period of cultivation and cereal cropping. Organic P was closely correlated with organic C and total N in these soils. The mean amounts of P contained in the grain and the dry matter of each cereal crop from 1981 and 1984 were 7 · 6±1 · 8 and 8 · 3±2 · 4 kg P/ha, respectively. Thus, most of the soil total P loss could be accounted for by crop removal, of which organic P contributed about 40%. However, the continuous decline in available P, especially below 15 mg P/kg soil, warrants remedial measures to arrest the decline in the yields of crops grown on these Vertisols.