Field and laboratory investigations into the effect of the parasitic isopod Anilocra pomacentri (Cymothoidae) on the population dynamics of the reef fish Chromis nitida (Pomacentridae) were carried Out at Heron Island, Great Barrier Reef, Australia. Fish carried a single adult parasite just posterior and dorsal to the eye either to the right or to the left of the midline. The adult parasite was overdispersed among fish on patch reefs (dispersion factor, k = 0·69). Sequential field observations on a single cohort of fish showed that parasites significantly depressed growth, reproduction, and survivor-ship. The von Bertalanffy growth coefficients (a measure of somatic growth) were 0·10 for parasitized fish compared with 0·17 for non-parasitized fish. Female fish carrying the parasite produced only 12% of the number of eggs produced by non-parasitized fish of the same size. In the field, the mortality of infected juvenile C. nitida (LCF 15–30 mm) was estimated to be at least 88% in the first 70 days after recruitment of the fish. The mortality of uninfected recruits over the same period was 66%. In laboratory trials, the mortality associated with the infection of juvenile fish by larval parasites ranged from 78% for small fish (mean LCF 15·0 mm) to 28% for larger fish (mean LCF 24·9 mm) within 4 days of experimental infection. This is one of the few studies that evaluates the effect of a parasite on a population of fish in the field.