The cleaning system is a classic model for cooperative behaviour among species in which cleaner fish or shrimps remove parasites and/or dead tissue from clients that pose at stations to solicit services from them (Cote 2000). Sharks, including pelagic thresher sharks, host a variety of ectoparasites that may affect their health and fitness (Oliver et al. 2011). While ectoparasitic digeneans have been observed to infect thresher sharks (Curran and Overstreet 2000), their association with cleaner fish diet has not been previously documented. In August 2013, light microscopy was used to examine ectoparasite specimens that were taken from the cloacas of dead pelagic thresher sharks caught in the central Visayas of the Philippines (N 11°19′, E 124° 11′), near a cleaning station that they are known to frequent (Oliver et al. 2011) (Fig. 1). The morphology of the parasites, and measurements carried out revealed that the samples belonged to the genus Paronatrema (Dollfus 1937) (Family: Syncoeliidae). Previous observations made at this site showed that cleaner wrasse (Labroides dimidiatus and Thalassoma lunare) preferentially forage in the pelvic region of thresher sharks (Oliver et al. 2011). Since no other species of parasite was found in this area of the dead thresher sharks, it appears likely that digeneans comprise an important part of cleaner fish diet. While the inclusion of helminths in the diet of cleaners is well documented (Cote 2000), this is the first instance in which digenea have been observed to contribute to the cleaner-client system.