Summary. The effect of particle size on the agronomic effectiveness of a moderately reactive phosphate rock (from the Hamrawein deposit in Egypt) was investigated at 4 of the field sites in the National Reactive Phosphate Rock Project. The aim of these experiments was to determine whether the agronomic effectiveness of this fertiliser was increased by removing particles greater than 0.5 mm in diameter (these coarser particles constituted 28% of the fertiliser). The more reactive North Carolina phosphate rock was used as a reference fertiliser. Soils from 3 of the field sites were also used for glasshouse experiments. The forms of Hamrawein phosphate rock used in these experiments were the less than 0.50 mm particle size fraction (as was used in the field experiments), the 0.075–0.15 mm fraction, and the less than 0.50 mm particle size fraction finely ground. Subterranean clover was used as the test plant and North Carolina phosphate rock and monocalcium phosphate were used as reference fertilisers. A series of laboratory experiments were also undertaken to investigate the effect of particle size. The agronomic effectiveness of Hamrawein phosphate rock was not affected by the different particle size treatments used in the field and glasshouse experiments. The Hamrawein phosphate rock was about equal in effectiveness to North Carolina phosphate rock at the high rainfall leaching site that had acidic sandy soil, and was less effective than North Carolina phosphate rock at the other 2 field sites. The Hamrawein phosphate rock was similar in effectiveness to North Carolina phosphate rock and monocalcium phosphate in the glasshouse experiments with the 2 low P-sorbing soils and was much less effective than North Carolina phosphate rock and monocalcium phosphate in the high P-sorbing soil. Laboratory studies found that there were minimal differences in the amount and rate of dissolution of different particle size fractions of the Hamrawein phosphate rock in moist incubated soil. Free carbonate, most likely in the form of dolomite, was found in all particle size fractions of the Hamrawein phosphate rock and was particularly concentrated in the particle size fractions less than 0.15 mm. This fine dolomite fraction may have suppressed the extent that Hamrawein phosphate rock dissolves in soil, and may have negated any potential beneficial effect that reducing particle size by sieving or by grinding may have had on the agronomic effectiveness of Hamrawein phosphate rock in the field and glasshouse experiments.