I study the role of a molecular cue, Sema3D, in axon guidance in the zebrafish. Sema3D belongs to a class of guidance molecules generally thought to be repulsive to axonal growth cones. Sema3D is expressed near several regions of the retinotectal pathway and is involved in proper guidance of the axons across the midline of the optic chiasm and in correct retinotopic mapping onto the tectal surface. I have used time-lapse imaging of labeled axons in intact, living embryos to identify growth cone behaviors at specific points of the optic pathway and how they are changed by manipulation of Sema3D expression. Through these studies, I have shown that Sema3D helps direct the formation of the crossed axon pathway at the chiasm. My current research focuses on identifying mechanisms involved in this process, especially the role of cell adhesion molecules in retinal axon guidance.