Grossly visible sarcocysts were seen in the skeletal muscles of 1 of 12 13-lined ground squirrels, Spermophilus tridecemlineatus tridecemlineatus, collected in Nebraska. The tissue cyst wall was up to 5.0 μm thick and contained spikelike projections. Transmission electron microscopy of tissue cysts revealed they were similar to Sarcocystis campestris Cawthorn, Wobeser, and Gajadhar, 1983, previously known only from experimental infections in Richardson's ground squirrel Spermophilus richardsonii. Prominent electron-dense bodies were observed lining the microfilaments present in the spikelike projections of the sarcocyst wall. This is the first report of S. campestris in a natural intermediate host and the first report of this parasite outside of Saskatoon, Canada.

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