ABSTRACT: The rate at which parasites (mainly gnathiid isopod larvae) were removed from fish by the cleaner wrasse Labroides dimidiatus was investigated. To examine the effect of this parasite removal on the parasites of fish, the number of parasites removedper individual host fish Hemigymnus melapterus per day was estimated and compared to the infection rate and abundance of gnathiids on H. melapterus. The study was conducted at Lizard Island, Great Barrier Reef, Australia, using a combinationof observations of the feeding rates of cleaners, estimates of how much time individual hosts spend being cleaned, cleaner fish stomach content analyses, and a gnathiid manipulation experiment. The frequency and duration of inspection by L.dimidiatus were measured to provide an estimate of the feeding rate. Individual L. dimidiatus spent on average 256 +/- 11 (SE) min d^-1 inspecting 2297 +/- 83 fish. L. dimidiatus consumed a large number of parasites (1218 +/-118, mainly gnathiid isopods) each day. The estimated predation rate by L. dimidiatus was 4.8 +/- 0.4 parasites per minute of inspection or 0.5 +/- 0.05 parasites per inspection. The infection rate of gnathiids onto fish was high, with reducedgnathiid loads (by about 50%) on fish returning to levels similar to control fish within 1 to 6 d. These high infection rates suggest that a significant proportion of gnathiids removed by cleaner fish are quickly replaced. The high predation rate relativeto the number of gnathiids on fish and their infection rate shows that cleaner fish have an effect on the abundance of gnathiids on fish.