Synchronous cultures of Chlamydomonas segnis Ettl. were treated with aminocarb (0.5, 5.0, 10.0, or 50.0 micrograms ml-1) at each specific phase of the cell cycle, namely, G1, S, G2, and M phases. The subsequent effects of some macromolecular products were assayed at the end of the M phase. Aminocarb treatments of 0.5 microgram ml-1 were at the no effects level on the parameters monitored. However, the higher concentrations of aminocarb tested, namely, 5, 10, and 50 micrograms ml-1, dramatically affected not only macromolecular syntheses but also some cell cycle events. Algistatic and algicidal effects were obtained with some treatments.