A microbial method to determine sulphate concentration in water was developed on the basis of sulphate-dependent acid phosphatase (APase) in whole cells of Thiobacillus thiooxidans. The activity of the APase was determined colorimetrically by using p-nitrophenylphosphate as substrate. The APase was activated by sulphate. A linear relationship was obtained between the activity of the APase and the concentration of sulphate in the range 0-0.6 mM. Therefore, the sulphate concentration was estimated from the APase activity, represented by the absorbance (A(400)) The microbial method was applied to the determination sulphate in water. The lower limit of detection was 0.02 mM, the relative standard deviation being 2% for 10 measurements on a standard sample. As for practical samples, which were taken from rain, river and tap water, good agreement was obtained between the values measured by the microbial method and those given by a conventional barium chloranilate method. The relative standard deviation was 2.1% for 12 measurements of tap water. The activity of the APase was stable over a period of more than 100 days when the cells were stored in 0.1 M sodium acetate/acetic acid buffer (pH 5.0) at 4 degrees C.