Time courses of the accumulation of chlorophyll (Chl) a, Chl b, of mRNA transcribed from the genes encoding the Chl a/b-binding proteins of the light-harvesting complexes (LHCs) (Lhc mRNAs) and LHC apoproteins have been investigated in Scenedesmus wild type (WT) and the mutant C-2A' upon transfer from darkness to light. The WT develops the complete photosynthetic apparatus in darkness, whereas C-2A' exhibits a light-dependent formation of Chls and functional chloroplasts. Accumulation kinetics of Chls and LHC apoproteins were paralleled in green (WT) and greening (C-2A') cells. Illumination induced a steady increase in these compounds which was only moderate in the WT but pronounced in C-2A'. Accumulation of Lhc mRNAs, however, was strongly enhanced by light in both WT and C-2A' cells and showed a transient time course with a maximum after 4 h illumination. Amounts of Lhc mRNAs and LHC approproteins were correlated only during the initial 4 h of greening of C-2A'. In green cells (WT and later stages of greening of C-2A'), amounts of Lhc mRNAs and LHC apoproteins diverged markedly. These findings and results provided by in-vitro translation of polysomal RNA lead to the following conclusion: the light-regulation of Lhc-gene expression in Scenedesmus takes place mainly at the transcriptional level in greening cells and at the post-translational level in green cells.