1. In the sex role reversed pipefish Syngnathus typhle females compete for males, and males are more choosy than females. Before mating, females display a temporary sexual ornament in the form of a lateral zigzag pattern, which is an amplification of their permanent colour pattern. 2. In experiments males showed no significant preferences for permanently contrast-rich females, nor for females matching closely with the background. 3. Females in physical contact with a male could potentially develop the ornament, but under female-female competition females were more likely to display it than otherwise. 4. The ornament accurately predicted female mating success. More ornamented females displayed more actively towards males and were found closer to males than were non-ornamented females. 5. Females under predation threat were less ornamented than un-threatened females, suggesting a reduced potential for male choosiness under threat. 6. We conclude that the female ornament serves both to attract partners (males) and repel competitors (females).