The effect of soil clay mineralogy on the efficiency of (NH₄)₂SO₄ in flooded rice was investigated in a greenhouse pot trial with four clayey soils of diverse clay mineralogies (x-ray amorphous, montmorillonite, vermiculite, beidellite). KCl (75 kg K ha^–1) and triple superphosphate (25 kg P ha^–1) were incorporated in the soil with and without (NH₄)₂SO₄ (100 kg N ha^–1) before transplanting 1-week-old IR-36 rice seedlings which were then grown to maturity under flooded conditions. Efficiency of (NH₄)₂SO₄ was inferred from the response of agronomic characteristics such as tiller number, height, grain and straw yields to NH₄ fertilization.