Three seismic refraction profiles were shot in the Mariana Trough in the vicinity of the proposed axial rift spreading center. One long east-west trending line was shot across the axial high region, the elevated portion of the trough associated with the rift zone. The two other lines were shot in a rough north-south orientation subparallel to the trend of the axial rift, on crusts 1 and 5--6 m.y. old. Comparison of the results of the ray-tracing analysis for these two profiles indicated that thinning of layer 2A (velocity of 3.3 km/s) and development of a distinct layer 2B (velocity of 5.3 km/s and development of a distinct layer 2B (velocity of 5.3 km/s) and layer 3A (velocity of 6.7 km/s) takes place with increasing crustal age, implying a process of crustal evolution. The presence of large-scale crustal faults was noted in several instances. In addition, a consistent but low upper mantle seismic velocity of 7.7 km/s was measured for all three lines. These characteristics correlate with those previously observed for other areas of young oceanic crust. When compared with major active spreading centers such as the Mid-Atlantic Ridge and East Pacific Rise, the Mariana Trough appears similar to the former in most respects. Both the Mid-Atlantic Ridge and Mariana Trough are slow spreading (1-2 cm/yr half rate) and are characterized by rough topography, an axial rift, and a thick layer2. Like the Mid-Atlantic Ridge, the Mariana Trough also has no low-velocity zone in the crust that could be interpreted as a magma chamber at the spreading center.