Cinnamic aldehyde has been identified as the active fungitoxic constituent of cinnamon (Cinnamomum zeylanicum) bark oil. The fungitoxic properties of the vapours of the oil/active constituent against fungi involved in respiratory tract mycoses, i.e., Aspergillus niger, A. fumigatus, A. nidulans A. flavus, Candida albicans, C. tropicalis, C. pseudotropicalis, and Histoplasma capsulatum, were determined in vitro as minimum inhibitory concentration (MIC), minimum lethal concentration (MLC), inoculum density sustained, and exposure duration for fungicidal action at MIC and higher doses, as well as effect of incubation temperatures on fungitoxicity. It is concluded that these inhalable vapours appear to approach the ideal chemotherapy for respiratory tract mycoses.