The crustal thickness and crustal and upper mantle structure along the rift valleys of three segments of the northern Mid-Atlantic Ridge with contrasting morphologies and gravity signatures are determined from a seismic refraction study. These segments lie between the Oceanographer and Hayes transforms and from north to south have progressively deeper axial valleys with less along-axis relief and smaller mantle Bouguer gravity lows. Major variations in seismic crustal thickness and crustal velocity and density structure are observed along these segments. The thickest crust is found near the segment centers, with maximum crustal thicknesses of 8.1, 6.9, and 6.6¿0.5 km, decreasing from north to south. However, the mean crustal thickness is similar for each segment (5.6¿0.4, 5.7¿0.4 and 5.1¿0.3 km). Near the segment ends, crustal thickness is 2.5 to 5¿0.5 km with no systematic variation from north to south. At segment ends, both crustal velocities and vertical velocity gradients are anomalous and may indicate fracturing and alteration of thin igneous crust and underlying mantle. Away from segment ends, the thickness of the upper crust is relatively uniform along axis (~3 km), although its internal structure is laterally heterogeneous (velocity anomalies of ¿0.6 km s-1 over distances of 5 km), possibly related to the presence of discrete volcanic centers. The along-axis crustal thickness variations are primarily accommodated in the lower crust. The center of the northern segment (OH-1) has an unusually thick crustal root (excess thickness of 2--4 km and along-axis extent of 12 km). Our results are consistent with an enhanced supply of melt from the mantle to the segment centers and redistribution of magma along axis at shallow crustal levels by lateral dike injection. Along this portion of the Mid-Atlantic Ridge, our results suggest that differences in axial morphology, seismic crustal thickness, and gravity anomalies are correlated and the result of variations in melt flux from the mantle. A surprising result is that the melt flux per segment length is similar for all three segments despite their different morphologies and gravity signatures. This argues against excess melting of the mantle beneath segment OH-1. Instead, we suggest that the thickened crust at the segment center is a result of focusing of melt, possibly due to the influence of the thermal structure of the Oceanographer fracture zone on melt migration in the mantle. ¿ 2000 American Geophysical Union