The green microalga Botryococcus braunii produces large amounts of liquid hydrocarbons and is classified into three races, A, B, and L depending on the types of the hydrocarbons. The B race produces two types of triterpenoid hydrocarbons, methylated squalenes and botryococcens, which are promising as renewable fuels. In order to understand isoprenoid biosynthesis by the B race of B.braunii, the incorporation of ¹³C labelled glucose into tetramethylsqualene and C₃₀botryococcene was studied by quantitative ¹³C NMR spectroscopic analyses. The resulting labelling patterns showed that the isoprene units of those triterpenoids are derived from the methylerythritol phophate (MEP) pathway. We also isolated and characterized a gene for 1-deoxy-D-xylulose 5-phophate synthase (DXS) from this alga which catalyzes the first step in the MEP pathway.