Aerial material of Prostanthera aff. melissifolia and P. rotundifolia yielded three sesquiterpenes, two known compounds and a novel sesquiterpene, prostantherol. Prostantherol was identified as (rel)-1a alpha,3a beta,7a alpha,7b alpha-decahydro-4 beta-hydroxy- 1,1,4 alpha,7 alpha-tetramethylcyclopropa[a]-naphthalene on the basis of a comprehensive spectroscopic analysis. All three compounds were investigated for antimicrobial activity against phytopathogens.