Terpene derivatives converted by microbial biotransformation constitute an important resource for natural pharmaceutical, fragrance, and aroma substances. In the present study, the monoterpene a-phellandrene was biotransformed by 16 different strains of microorganisms (bacteria, fungi, and yeasts). The transformation metabolites were initially screened by TLC and GC/MS, and then further characterized by NMR spectroscopic techniques. Among the six metabolites characterized, 6-hydroxypiperitone, a-phellandrene epoxide, cis-p-menth-2-en-1-ol, and carvotanacetone, which originated from (-)-(R)-a-phellandrene, are reported for the first time in this study. Additionally, the substrate and the metabolite 5-p-menthene-1,2-diol were subjected to in vitro antibacterial and anticandidal tests. The metabolite showed moderate-to-good inhibitory activities (MICs=0.125 to >4 mg/ml) against various bacteria and especially against Candida species in comparison with its substrate (-)-(R)-a-phellandrene and standard antimicrobial agents.