Grindelia squarrosais an arid lands herb that has been used in Native American traditional medicine, is a potential source of pharmacologically active compounds, and has been explored as a source of biofuel. The purpose of this work was to examine the essential oil composition ofG. squarrosafrom southern Idaho. Gas chromatographic methods revealed the essential oil ofG. squarrosavar.serrulatato be rich in monoterpenoids, α-pinene (21.9%), limonene (17.1%), terpinolene (10.6%), and borneol (6.5%). The essential oil composition ofG. squarrosafrom Idaho is similar to that previously reported from specimens collected from Montana and confirms the volatile phytochemistry of plants growing in North America. The major essential oil components were screened for antimicrobial activity against respiratory and dermal pathogens. (–)-β-Pinene showed strong antibacterial activity againstStreptococcus pneumoniae(MIC 39.1 μg/mL) and (–)-borneol showed strong activity againstStaphylococcus aureus(MIC 78.1 μg/mL).