THE small-scale segmentation of mid-ocean ridges with an axial rise has been modelled by considering each ridge segment as a giant crack in the lithosphere with a tip propagating along the ridge axis(1,2). For ridges with an axial valley, however, this type of model fails because the lithosphere is too thick to tear(3). Yet such ridges are clearly segmented, as defined by morphology, gravity and structure at the 50-100 km length scale. The-ridge offsets are large(4,5), and vary dramatically with time. This type of segmentation is commonly related to a three-dimensional, small-scale mantle flow occurring in the partially molten asthenosphere below the ridge(6-8). Here we propose a model for segmentation in such ridges, in which the convective flow below the ridge favours an asymmetrical breaking of the axial-valley lithosphere. This leads to the development and separation of ridge segments in a pattern that mimics the observed geometry and temporal evolution of the segmentation of most ridges,vith an axial valley, If our model is correct, it implies that coupling of oceanic lithosphere to small-scale convection controls the dynamics of mid-ocean ridges.