The stereotyped mating behaviour of the Caenorhabditis elegans male is made up of several substeps: response, backing, turning, vulva location, spicule insertion and sperm transfer. The complexity of this behaviour is reflected in the sexually dimorphic anatomy and nervous system. Behavioural functions have been assigned to most of the male-specific sensory neurons by means of cell ablations; for example, the hook sensory neurons HOA and HOB are specifically required for vulva location. We have investigated how sensory perception of the hermaphrodite by the C. elegans male controls mating behaviours. Here we identify a gene, lov-1 (for location of vulva), that is required for two male sensory behaviours: response and vulva location. lov-1 encodes a putative membrane protein with a mucin-like, serine-threonine-rich amino terminus followed by two blocks of homology to human polycystins, products of the autosomal dominant polycystic kidney-disease loci PKD1 and PKD2. LOV-1 is the closest C. elegans homologue of PKD1. lov-1 is expressed in adult males in sensory neurons of the rays, hook and head, which mediate response, vulva location, and potentially chemotaxis to hermaphrodites, respectively. PKD-2, the C. elegans homologue of PKD2, is localized to the same neurons as LOV-1, suggesting that they function in the same pathway.