Most of the LJ 2S+1 multiplets of the Nd3+ ion (4 f3) in Bi4 Ge3 O12 and Bi4 Si3 O12 single crystals have been experimentally observed in the UV, visible, and NIR spectroscopic regions. The majority of the Stark energy levels were identified for each multiplet, including some recently identified. In these crystals, the Nd3+ ions substitute Bi3+ ions at sites of C3 point group symmetry. A Hamiltonian including the Coulomb, spin-orbit, interconfigurational interaction terms of the Nd3+ (4 f3) configuration, and trigonal crystal-field terms was used to fit these energy levels. The Hamiltonian was diagonalized in the SLJMJ complete 4 f3 basis that includes 364 states. A least-squares fit was carried out for all the observed energy levels. Previously obtained Bkq crystal-field parameters did not reproduce all of the observed Nd3+ energy levels. The Bkq parameters obtained in this work allow a good energy level fitting for both lattices. The obtained crystal-field parameters produced an average standard deviation of about 20 cm-1. Although the intensity of the crystal field experimented by the neodymium ions in both lattices is quite similar, an analysis of the nephelauxetic ratio shows a larger wave-function overlap in bismuth silicate than in bismuth germanate. Also, the spectroscopic electron paramagnetic resonance Landé g factors were calculated for both lattices, obtaining excellent agreement with the experimental values. © 2007 The American Physical Society.