Synchronous cultures of Chlorella pyrenoidosa (strain 7- 11-05) have been utilized to measure the potential expression of the structural gene for isocitrate lyase (three D_S-Isocitrate glyoxylate-lyase, EC 4.1.3.1) during the cell cycle. Synthesis of the enzyme could be induced by placing cultures in the dark on acetate, with the induction process occurring in a quadratic fashion. By addition of cycloheximide during the course of induction, the increase in isocitrate lyase activity was shown to result from de novo protein synthesis. In the absence of protein synthesis the enzyme was stable for at least five hours. The pattern of uninduced isocitrate lyase synthesis during the cell cycle in continuous light, paralleled the stepwise increase of total cellular DNA. The enzyme appeared to be fully repressed for most of the cell cycle, and was derepressed during the time of DNA replication. Isocitrate lyase could be induced at all times in the cell cycle, indicating that the potential for gene expression is continuous in this eukaryote. A time lag was observed between the beginning of DNA replication and the initial rise in potential for isocitrate lyase gene expression. The control of gene expression in Chlorella appeared to be similar to that found in a fission yeast. / Ph. D.