Transmission studies with Sarcocystis idahoensis of deer mice (Peromyscus maniculatus) and gopher snakes (Pituophis melanoleucus) were conducted to determine host specificity of various stages of the parasite. Sporocysts were not passed by four dogs or four cats fed infected skeletal muscle from deer mice. Seven white mice (Mus musculus) and 34 white-footed mice (Peromyscus leucopus) were negative for sarcocysts and liver meronts following oral inoculation with S. idahoensis sporocysts; however, excystation of sporocysts occurred in two white-footed mice killed four hours post inoculation (PI). A gopher snake orally inoculated with sporocysts remained negative for coccidia for two months PI. Three deer mice orally inoculated and three intraperitoneally (IP) inoculated with tachyzoites from liver meronts developed sarcocysts in their skeletal muscles similar to those seen in deer mice orally inoculated with sporocysts. Liver meronts were not found. Ten deer mice orally inoculated and 10 deer mice inoculated IP with bradyzoitee from S. idahoensis sarcocysts remained negative for sarcocysts and liver meronts at necropsy 17 days PI.