THE process by which basaltic melt is generated and extracted beneath mid-ocean ridges is poorly understood. Knowledge of the distribution of melt within the parent mantle peridotite during the early stages of melting is important for interpretation of geophysical experiments and for construction of models of magma coalescence and extraction(1-4). Static experiments on mantle rocks and selected analogue materials have shown that, for small melt fractions, melt is concentrated along three-grain intersections, forming an interconnected web of tubes(5-9). Low-pressure deformation experiments on olivine + melt specimens have yielded the same conclusion(10,11). But in similar experiments on salt-brine mixtures during ductile deformation, the fluid emerges from the triple junctions where it resides under static conditions and spreads onto grain boundaries(12-16). Here we report the results of low-stress deformation experiments on partially molten peridotite at mantle temperatures and pressures, which show that such dynamic melting produces microstructures analogous to those of the salt-brine experiments. The very low viscosity of these specimens suggests that in the Earth, dynamic wetting could lead to melt separation at very low melt fractions, and to melt focusing at ridges.