The present invention provides nanoparticles having a core/shell structure consisting of a core comprising a Group III element and a Group V element at a molar ratio of the Group III element to the Group V element in the range of 1.25 to 3.0, and a shell comprising a Group II element and a Group VI element and having a thickness of 0.2 nm to 4 nm, the nanoparticles having a photoluminescence efficiency of 10% or more and a diameter of 2.5 to 10 nm; a method of producing the water-dispersible nanoparticles comprising bringing a dispersion of III-V semiconductor nanoparticles in an organic solvent into contact with an aqueous solution of a Group II element-containing compound and a Group VI element-containing compound to thereby transfer the III-V semiconductor nanoparticles of the organic solvent dispersion to the aqueous solution, and then irradiating the aqueous solution with light; and a method of producing a glass matrix having the nanoparticles dispersed therein. The present invention provides III-V semiconductor nanoparticles having a high photoluminescence efficiency in an aqueous solution, and a method of producing the nanoparticles. The invention further provides a fluorescent material with high PL efficiency containing the III-V semiconductor nanoparticles retained in a glass matrix, a method of producing the fluorescent material, and a light-emitting device containing the fluorescent material.