In the Mediterranean Sea, the most conspicuous cleaner fish is the wrasse, Symphodus melanocercus, a specialized cleaner wrasse. In this fish species, energetic investments in reproduction are different for males and females. Female S. melanocercus were found to be more active in cleaning and ingested significantly more gnathiid isopod larvae than males. Male cleaners foraged more on benthic animals and showed a strong preference for clients according to their mucus characteristics. Results indicate that female S. melanocercus are more likely to be specialized, in terms of cleaning behaviour, than males. We suggest that females are more honest than males and that they play an important role in preserving the good reputation of S. melanocercus as a cleaner fish.