The importance of contact and diffusible pheromones in the reproductive biology of the harpacticoid Tigriopus japonicus was studied. When given a choice, males preferred developmentally advanced conspecific female partners over less mature or congeneric females. Males judged female attractiveness on a relative scale, based on the locally available females. The attractiveness of a female copepodid was reduced with non–fatal proteolytic treatment, but only if normal females were also present. To sample the available females, males repeatedly grabbed the caudal rami and terminal urosome segment of potential partners before committing themselves to guarding one female. Males occasionally dropped their copepodid partners. Releases increased in frequency in water conditioned from virgin adult females and adult males, decreased in mated–female conditioned water and were unaffected by copepodids or their treated water. The waning attractiveness of a recently mated female was tracked over 16 h. Relationships between Tigriopus japonicus adults appeared to involve both contact and diffusible pheromones. No evidence of a diffusible copepodid pheromone was uncovered.