Macroscopic gastrointestinal sarcocysts were detected in 36 of 270 (13%) Tasmanian pademelons (Thylogale billardierii) and 47 of 292 (16%) Bennetts wallabies (Macropus rufogriseus) from January 1995 to March 1996 at onshore and offshore study sites in Tasmania (Australia). The sarcocysts were characterized using light and electron microscopy. The ultrastructure of the primary cyst wall was consistent with that of Sarcocystis mucosa, an apicomplexan parasite commonly found in macropodid marsupials. Although conventional statistical tests were not applied to data, there were apparent differences in the prevalence of infection in macropodid marsupials inhabiting onshore (19%) versus offshore (0%) sites. These differences were attributed to the presence or absence of medium-sized dasyurid marsupials. The results of this study provide strong circumstantial evidence that these dasyurid marsupials are the probable definitive hosts for S. mucosa in free-ranging Tasmanian macropodids.