Freshwater fishes are the usual hosts of glochidia (the parasitic larval stage) of freshwater mussels (Bivalvia: Unionoidea). Westralunio carteri Iredale 1934 (Carter’s mussel), the only unionoid species found in the Southwest Coast Drainage Division of Australia, is endemic to the region and is currently listed as Vulnerable on the IUCN Red List, yet nothing is known of its host species. Small, white, bladder-like cysts were observed macroscopically on Tandanus bostocki Whitley, 1944 (freshwater cobbler) captured from the Blackwood River, Western Australia. Light microscopy of sectioned cysts revealed that they contained glochidia and these were of similar size and shape to glochidia obtained from gravid females of W. carteri. Glochidia were found on 40.7% of 461 T. bostocki examined, with a mean intensity of 10.6 cysts per infested fish. Prevalence of infestation was greater on juvenile than on adult fish. The findings represent an important step in developing conservation measures for W. carteri in this region.