The molecular controls governing organ shape are poorly understood. In the nematode Caenorhabditis elegans, the gonad acquires a U-shape by the directed migration of a specialized ＇leader＇ cell, which is located at the tip of the growing gonadal ＇arm＇(1). The gon-1 gene is essential for gonadal morphogenesis: in gon-1 mutants, no arm elongation occurs and somatic gonadal structures are severely malformed(2). Here we report that gon-1 encodes a secreted protein with a metalloprotease domain and multiple thrombospondin type-1-like repeats. This motif architecture is typical of a small family of genes that include bovine procollagen I N-protease (P1NP), which cleaves collagen(3), and murine ADAMTS-1, the expression of which correlates with tumour cell progression(4). We find that gon-1 is expressed in two sites, leader cells and muscle, and that expression in each site has a unique role in forming the gonad. We speculate that GON-1 controls morphogenesis by remodelling basement membranes and that regulation of its activity is crucial for achieving organ shape.