The roles of six carnivores as potential sources to sheep of the sarcosporidial parasites causing cysts in meat were compared in a series of experiments carried out between 1973 and 1980. The research was concomitant with other studies that confirmed the prey-predator-prey-predator cycle of transmission. Infected carnivores act as vectors, excreting in their faeces coccidial sporocysts infective to the meat animal. For 60 experimental infections, sheep meat containing sarcocysts or sarcocysts removed from sheep meat were fed to experimental carnivores. Faecal samples were examined for sporocysts over 60 days post infection. Neither macroscopic (visible) sarcocysts nor microscopic sarcocysts from sheep carcasses were transmissible by humans or by ferrets (domestic polecats). Microscopic sarcocysts were readily infective to dogs, both domestic and dingo, as well as to foxes, but not to cats. Large numbers of sporocysts that could contaminate the environment were excreted. Both fat and thin visible sarcocysts were transmitted exclusively to cats. Although cats responded with relatively low levels of sporocysts, these were considered adequate to provide sufficient pasture contamination for the life cycle to be perpetuated. It is concluded that, while domestic dogs may be the greatest source of infection for sheep with microscopic sarcocysts, foxes as vectors also pose a threat to sheep production. Similarly, due to their widespread presence, feral cats play a role as well as domestic or semi-domestic cats in the spread of sarcocysts causing visible carcass lesions.