Presynaptic and postsynaptic membranes directly oppose each other at chemical synapses, minimizing the delay in transmitting information across the synaptic cleft. Extrasynaptic neuronal surfaces, in contrast, are almost entirely covered by processes from glial cells. The exclusion of glial cells from the synaptic cleft, and the long-term stability of synapses, presumably result in large part from the tight adhesion between presynaptic and postsynaptic elements. Here we show that there is another requirement for synaptic maintenance: glial cells of the skeletal neuromuscular synapse, Schwann cells, are actively inhibited from entering the synaptic cleft between the motor nerve terminal and the muscle fibre. One inhibitory component is laminin 11, a heterotrimeric glycoprotein that is concentrated in the synaptic cleft. Regulation of an inhibitory interaction between glial cells and synaptic cleft components may contribute to synaptic rearrangements, and loss of this inhibition may underlie the loss of synapses that results from injury to the postsynaptic cell.