A 2-week model life cycle of the green alga Haematococcus pluvialis was constructed, consisting of four cell stages : vegetative cell growth, encystment, maturation, and germination. Each algal cell stage could be distinguished by the ratio of pigments (carotenoid/chlorophyll) and the intracellular protein content. Using the culture system developed, light was shown to be essential for both carotenogenesis and cell differentiation (encystment and germination). The results also suggested that H. pluvialis has a novel photosynthesis-dependent system of carotenogenesis regulation.