coral reefs harbour an extraordinary, concentrated diversity of life. What are the implications of this for parasites? After the corals themselves, the most striking component of coral reefs is the fishes. Individual coral reefs may harbour as many as a thousand species of fishes. Like most fishes, those of coral reefs bear remarkable loads of parasites. Records of digenean trematodes from 214 species of fishes from the Great Barrier Reef and 103 species of fishes from inshore Australian waters are compared to examine the ecological expression of parasite diversity on coral reefs. Coral reef fish had an overall prevalence of infection of 70% compared with 48% for their inshore counterparts and averaged 2.61 species of digenean per host species as compared with only 1.41 for the inshore group. A total of 236 species of Digenea has been collected from reef fishes. Most of the digeneans are concentrated in just a few families. Host-specificity of digeneans of reef fishes is variable but, on average, each species infects 2.37 host species. Characteristics of the fauna studied so far are used to predict that the 1300 fishes of the Australian Great Barrier Reef are likely to harbour some 2270 species of Digenea.