The essential oil ofZingiber cassumunar(Plai oil) exhibits antimicrobial activity against a wide range of Gram-positive and Gram-negative bacteria, dermatophytes and yeasts. Dermatophytes were found to be the most susceptible microorganisms followed by yeasts, whereas bacteria were the least susceptible. The mean diameter of the inhibition zone determined by the disc diffusion screening method increased with increasing Plai oil concentration between 6.25 and 50 vol %. The minimum bactericidal concentration (MBC) determined by the broth macrodilution method ranged from 0.62 to 2.5 vol % for Plai oil and from 52 to 79 mg/mL for the 5 wt % Plai oil gel, whereas the minimum fungicidal concentration (MFC) ranged from 0.31 to 1.25 vol % for Plai oil and from 13.8 to 39.5 mg/mL for the 5% Plai oil gel. Copyright © 2006 John Wiley & Sons, Ltd.