Abstract Allophanate lyase can be induced by urea or acetamied 20–40-fold within 4 h in NH 4 +-deprived cultures of Chlamydomonas reinhardi. In light-synchronized cultures, allophanate lyase induction appeared to be limited to the light phase of the cell cycle, provided that culture samples were induced under ongoing illumination conditions (i.e. light induction of light phase cells and dark induction of dark phase cells). However, when culture samples were induced under constant light conditions this cell cycle pattern was abolished. Light was found to be required for allophanate lyase induction and this was shown to be due, in part, to light requirement for inducer uptake. The relationship between allophanate lyase induction and gametogenesis is discussed.