The minimal residual nuclear structure, which is referred to as the nuclear matrix, was isolated from whole sea urchin embryos of the species Paracentrotus lividus. Electron microscopy of the blastula and pluteus nuclear matrices revealed structures which preserved the spherical shape of intact nuclei. When compared with the pluteus nuclear matrix, the blastula matrix was found to retain significantly more nuclear material. Evidence is presented for a stage-specific composition of the nuclear matrices of sea urchin embryos along with preliminary results pointing to an involvement of some of the nuclear matrix proteins in the formation of the chromatin complex.