Semaphorins/collapsins are a large family of secreted and cell surface molecules that are thought to guide growth cones to their targets. Although some members are clearly repulsive to specific growth cones in vitro, the in vivo role of many of these molecules in vertebrate embryos is still unclear. As a first step towards clarifying the in vivo role of semaphorins/collapsins, we analyzed semaZ1a in the simple and well-characterized zebrafish embryo. SemaZ1a is a secreted molecule that is highly homologous to Sema III/D/collapsin-1, and it can collapse chick dorsal root ganglion growth cones in vitro. It is expressed in highly specific patterns within the developing embryo, which suggests that it influences outgrowth by a variety of growth cones including those of the posterior lateral line ganglion. Consistent with this hypothesis, the peripherally extending growth cones of posterior lateral line neurons retract and partially collapse during normal outgrowth.