The onset of carotenogenesis in Chlorella vulgaris and the change in nature and concentration of pigments with time was studied. The succession of pigments observed was interpreted in terms of relative efficiencies of carotenoid interconversion pathways, and this might be used to monitor the progress of the carotenogenic process. This work is relevant to the use of dry Chlorella biomass, as a naturally encapsulated form of a natural colouring ingredient, in animal feed. Copyright (C) 1996 Elsevier Science Ltd.