A stimulator of teliospore germination was concentrated from thistle roots (Cirsium arvense) by steam distillation and extraction by hexane. Concentrations of 25 pL/L stimulated teliospore germination 50% in 7 days. Teliospores exposed to volatiles from crude extract at time intervals ranging from 1 min to 24 h reached a maximum germination plateau with 1 h or longer exposures when counted at 7 days. TLC, NMR, and GC-mass spectrometric analysis of the hexane extract indicated the presence of C17 unsaturated hydrocarbons with four, three, two, and one double bonds. 1-Pentadecene was also found. Most of the stimulatory activity occurred with the compound separated by TLC at Rt 0.26, shown to be (Z,Z,Z)-1,8,11,14-heptadecatetraene, or aplotaxene. The spot at Rf 0.60 contained 1-neptadecene and 1-pentadecene. These compounds were analyzed quantitatively by gas chromatography. Aplotaxene was found in thistle roots at concentrations of 0.032 pg/g of fresh roots. Synthetic aplotaxene was found to possess no stimulatory activity on Canada thistle rust teliospore germination.Canada thistle [Cirsium arvense (L.) Scop.] is a widely dispersed noxious weed that is difficult and expensive to control by herbicides. This plant is susceptible to a rust, Puccinia punctiformis, that is particularly devastating in the aecial stage. Infection is initiated from basidiospores produced by germinating teliospores. Turner et al. (1982) reported a biologically active substance from Canada thistle roots that stimulated teliospore germination.