Attempts were made to transmit to clean voles a species of Sarcocystis found in the muscle of the European short-tailed vole Microtus agrestis, by inoculation of bradyzoites, feeding of infected muscle and feeding of faeces collected from voles and cats fed on infected vole flesh. Transmission was not achieved. The results are discussed in the light of the positive findings reported in the older literature with other rodent Sarcocystis spp. and the recent elucidation of the coccidian life-cycle of several species of Sarcocystis. In view of the reported successful faecal transmission of Sarcocystis of Mus musculus, mouse-to-mouse by "coccidia-like bodies" and cat-to-mouse by sporocysts and by oocysts, it is suggested that more than one species of Sarcocystis involving the same or different final hosts may infect a single rodent species. It is furthermore speculated that the Sarcocystis species of Microtus, with which this paper is mainly concerned, will be found to undergo a coccidian sexual developmental cycle in the intestinal tissue of wild vole predators, e.g. mustelids or owls.