We assayed biologically significant isoprenoids for antibacterial and antifungal activities. Isoprenoids varied widely with respect to their growth inhibitory activities towards Gram-positive and Gram-negative bacteria and fungal species. Of particular interest was the finding that geraniol acted as a broad spectrum of antibiotic, and significantly inhibited the growth of Gram-positive Staphylococcus aureus 209P and methicillin resistant S. aureus 834, Gram-negative Escherichia coli IFO-3806, and Shigella sonnei, as well as the fungus Trichopyton sp. Farnesol inhibited S. aureus strains 209P and 834 to a greater extent than did geraniol, but did not detectably inhibit Gram-negative bacteria.