The composition of the diet of the cleaner wrasse Labroides dimidiatus was quantified at different times and at tao spatial scales. I examined feeding selectivity in the cleaner fish by comparing its diet to the parasite: load of seven species of fish hosts it regularly cleans. Parasitic gnathiid isopods made up 50% of the estimated food volume and 95% of the prey items of Labroides dimidiatus at Lizard Island, Australia. The remaining 50% of the food material mainly was unidentified digested organic matter. Tnf greatest difference in the diet occurred between Heron and Lizard Islands, with only small differences among sites at the latter The total biomass of gnathiid isopods in the diet was four times higher at Lizard Island than at Heron Island, Fish from Lizard Island contained more caligid larvae and other parasitic copepods in their diet, whereas fish from Heron Island contained more nonparasitic copepods and mucus. The number and estimated biomass of gnathiids in the diet more than doubled from May to January. Variation in the diet implies spatial and temporal flexibility in the foraging patterns of L. dimidiatus. The higher ratio of gnathiids to other crustacea in the diet compared to that on host fish, suggests that L. dimidiatus is a selective feeder.