The seasonal variation in the chemical composition of the leaf essential oil of Zanthoxylum clava-herculis has been analyzed by GC-MS. Three individual trees were sampled four times during the course of the 2004 growing season. Notable differences were recorded in the essential oil yields from the three trees on the four collection dates; yields were highest in May and lowest in July. The leaf essential oils were made up of 25 components, largely menthane monoterpenoids, dominated by limonene (44-73%) and 1,8-cineole (16-43%), with lesser amounts of alpha-thujene, linalool, gamma-terpinene, and alpha-terpineol. The ratio of oxygenated monoterpenoids to monoterpene hydrocarbons generally increased during the season, largely reflected in the 1,8-cineole/limonene ratio.