The composition of the essential oil of Zingiber cassumunar Roxb. from Indonesia was examined by GC and GC-MS. A major part of the oil consists of monoterpenes with sabinene and terpinen-4-01 as main constituents. Sesquiterpenes accounted for a small part of the oil with sesquiphellandrene being the principal constituent. In addition to these terpenes the oil contains a number of phenylbutanoids. The essential oil obtained by hydrodistillation contained about 25 % of these phenylbutanoids whereas the oil obtained by extraction with light petroleum had about 46 %, with trans-l-(3,4-dimethoxyphenyI)but-l-ene, trans-l-(3,4-dimethoxyphenyI)butadiene and trans-4-(3,4-dimethoxyphenyl)but-3-ene-l-y1 acetate as the main constituents.