Connectin, a cell surface protein of the leucine-rich repeat family in Drosophila, is expressed on the surface of a subset of embryonic muscles (primarily lateral muscles), on the growth cones and axons of the motoneurons that innervate these muscles (primarily SNa motoneurons), and on several associated glial cells. When coupled with its ability to mediate homophilic cell adhesion, these results led to the suggestion that Connectin functions as an attractive signal for SNa pathfinding and targeting. In the present study, we ectopically expressed Connectin on ventral muscles normally innervated by SNb motoneurons. The SNb growth cones change both their morphology and their trajectory when they encounter ectopic Connectin-positive ventral muscles, displaying "bypass," "detour," and "stall" phenotypes. Moreover, SNb synapse formation is prevented by Connectin expression on ventral muscles. These results reveal a repulsive function for Connectin during motoneuron growth cone guidance and synapse formation.