ABSTRACT Stage-characterized populations of dark-grown heterotrophs of an asexually reproducing, unicellular alga, Chlorella mutant C-10, have been studied to determine light-induced ultra-structural changes in plastids and in chlorophyll synthesis. C-10 requires light to synthesize chlorophyll and to produce a photosynthetically potent chloroplast. Thylakoid development is followed through a series of stages from prethylakoidal tubules to primary thylakoids which produce blebs. The blebs expand and slide over the primary thylakoid, and then form a secondary thylakoid; the process continues until the characteristic stacks of tertiary thylakoids are present in the mature chloroplast. A dramatic decrease in the rate of chlorophyll synthesis (‘ early-rate-decline’) is observed approximately 1 h after the onset of induction. This early-rate-decline correlates with the appearance of blebs on the primary thylakoid. Twelve hours after the onset of induction, the photosynthetically impotent proplastid develops into a mature plastid containing stacks of disc-like thylakoids. The application of chloramphenicol, puromycin and actinomycin D suggests that the early-rate-decline is associated with an insufficient rate of synthesis of the holochrome proteins.