The Ayu Trough lies on the southern boundary between the Philippine Sea Plate and Caroline Plate. Although this trough may be the best place to study the evolution and kinematics of the Philippine Sea Plate, the origin and evolution of this trough are poorly understood. Our geophysical and morphological surveys in the northern part of the Ayu Trough revealed that the trough shares morphological similarities with slow-spreading mid-ocean ridges. The seafloor ages and an average spreading rate of the trough were inferred from the average length of the ridge segments, distribution of sediment thickness, and basement subsidence. Based on the seismic section at 3¿30'N, the opening of the trough started about 25 Ma with an average half-spreading rate 4.1 mm/yr. This spreading rate is much slower than some previous estimates. The present magma production rate and activity of the trough is estimated to be even lower than the 25 m.y. average. The seafloor depth of the trough axis and relief of the axial valley are approximately twice as deep as that of active mid-ocean ridges. The trough axis valley is covered with sediment in contrast with the axial valleys of active mid-ocean ridges. Sedimentation at the trough axis shows that the spreading rate has been quite slow for the last 2.5 Ma. ¿ American Geophysical Union 1995