The European corn borer consists of two sex pheromone races, leading to strong reproductive isolation which could represent a first step in speciation. Female sex pheromone production and male behavioural response are under the control of different genes, but the identity of these genes is unknown. These authors show that allelic variation in a gene essential for pheromone biosynthesis accounts for the phenotypic variation in female pheromone production, leading to race-specific signals.