Cleaner fish are stocked into salmon farms to prey on salmon lice Lepeophtheirus salmonis , acting as a form of continuous parasite control. Little is known about how lice adapt to this form of biological control, although anecdotal observations suggest that lice populations in salmon cages exposed to cleaner fish are smaller, lighter, and concentrated on the head region of the host. Here, we tested whether cleaner fish selected for lice size, pigmentation level, and position on the fish. We stocked tanks with adult lice-infested Atlantic salmon Salmo salar and either ballan wrasse Labrus bergylta or lumpfish Cyclopterus lumpus . After 60-80% of lice were removed by cleaner fish, the remaining parasites were assessed for size, pigmentation, and position on the fish, and were compared to lice on salmon in tanks without cleaner fish. Remaining lice after exposure to lumpfish were 19% smaller, but pigmentation levels and position on the fish were similar to controls. After exposure to ballan wrasse, remaining lice were 11% smaller, 12% less pigmented, and their distribution had shifted toward the head. Our results demonstrate that cleaner fish select for lice traits linked to detectability, although the direction and strength of selection differ between species. The extent of selective effects will depend on the relative deployment and efficacy of each cleaner fish species under commercial conditions, and how other widely used control measures act upon these traits. Here we demonstrate that selective pressure by cleaner fish exists, with less detectable lice, including those differing in pigmentation, size, or preferred position on the fish, being more likely to survive when exposed to cleaner fish. Future studies should assess heritability through multi-generational experiments to determine whether the traits that shifted in response to selection are heritable.