What is to our knowledge the first high-density (>10(12) bits/cm(3)) optical recording of digital information in a multilayered, three-dimensional format is reported. Information is written as submicrometer volume elements of increased refractive index in a photopolymer by two-photon excitation of a photoinitiator at the waist of a highly focused beam from a colliding-pulse mode-locked laser. Quadratic dependence of two-photon excitation on intensity confines polymerization to the focal volume. Information is read with sufficient axial resolution by differential interference contrast microscopy. This write-once, read-many technique should increase the capacity of the spinning disk format by 100-fold.