Botryococcus braunii is a green, colonial microalga that produces large amounts of hydrocarbons. B. braunii B70 was estimated to be B race by the incorporation of radioactivity from L-[methyl(14)C]-methionine into hydrocarbon. The hydrocarbon-synthesizing activity of B70 cells was determined by feeding experiments using C-14-compounds. (NaHCO3)-C-14 incorporation rate into the hydrocarbon was high in the early logarithmic growth phase but it declined thereafter. Hydrocarbon-synthesizing activity from [2-C-14] pyruvate in 15-day cells was 80% of that in 5-day cells. In contrast, hydrocarbon-synthesizing activity from (NaHCO3)-C-14 and L-[methyl(14)C]-methionine decreased remarkably by 15 days after inoculation. Hence, the allocation of carbon was a regulatory step in hydrocarbon biosynthesis during the early logarithmic growth phase. The high activity of pentose phosphate pathway in the early logarithmic growth was seemed to be the contribution of the supply of NADPH for botryococcene synthesis. (C) 2011 Elsevier Ltd. All rights reserved.