The invention concerns a colloidal dispersion of a phosphate of a rare earth and a process for its preparation. The dispersion is characterized in that it comprises anisotropic and disaggregated or disaggregatable particles of a phosphate of at least one rare earth and an anion of a monobasic acid, soluble in water and with a pKa of at least 2.5. It is prepared by a process in which a solution of a salt of at least one rare earth is mixed with phosphate ions while controlling the pH of the reaction medium to a value in the range 4 to 9 and in the presence of a monobasic acid, soluble in water and with a pKa of at least 2.5; the mixture obtained optionally undergoes a maturing step; the precipitate is then separated from the reaction medium; and said precipitate is then dispersed in water.