In Drosophila, motoneuron growth cones initially probe many potential muscle targets but later withdraw most of these contacts to form stereotypic synapses with only one or a few muscles. Prior to synapse formation, Fasciclin II (Fas II) is expressed at low levels on muscle. During synapse formation, Fas II concentrates at the synapse and disappears from the rest of the muscle. We previously showed that Fas II is required both pre- and postsynaptically for synaptic stabilization. Here, we show that the differential expression of target-derived Fas II has a profound influence on the patterning of synapse formation. A transient increase in muscle Fas II stabilizes growth cone contacts and leads to novel synapses that are functional and stable; targets that normally receive two inputs can now receive up to six inputs. Changing the relative levels of Fas II on neighboring muscles leads to dramatic shifts in target selection.