Rock phosphates from Udaipur (India), North Carolina, Florida, Tennessee and Missouri (USA) were acidulated with HCl or H2SO4, to the extent of 25, 50, 75 or 100 per cent of the requirement for complete conversion into single superphosphate. Partial acidulation resulted in the formation of a mixture of water- and citrate-soluble and -insoluble phosphates, the proportion of the first two increasing with increasing degree of acidulation, at the expense of the insoluble phosphate. For a given degree of acidulation, the P compounds formed with HCl or H2SO4 were of comparable composition, except that the HCl-acidulated products were hygroscopic. Pot experiments with acid P-deficient soils showed that the behaviour of the HCl- or H2SO4- acidulated products in respect of P availability in soil, grain yield response and P uptake by rice was more or less similar. Partially acidulated rock phosphate to the extent of 50 per cent with either of the acids was found to be suitable for growing rice under flooded soil conditions. There is thus scope for the use of HCl- acidulated rock phosphate as a substitute for the H2SO4- acidulated product, for growing rice in acid soils. The scope for use of lower degree HCl-acidulated product in an upland crop — rice rotation in acid soils is also discussed.