The combination of nitrogen deprivation and increased cultivation light intensity resulted in the synthesis of secondary carotenoids in flagellates of Haematococcus lacustris. The pigment pattern was characterized by high-pressure liquid chromatography analysis during the accumulation period and in response to inhibitors of carotenoid biosynthesis (diphenylamine, norflurazon and tetcyclacis). Diphenylamine treatment resulted in (i) a decrease in ketocarotenoids and (ii) an accumulation of β-carotene and zeaxanthin due to inhibition of the β-carotene oxygenase. Our results indicate that astaxanthin synthesis in H. lacustris follows the biosynthetic pathway elucidated in the marine bacterium Agrobacterium aurantiacum.