Chlamydomonas reinhardi and Chlorella ellipsoidea reproduce through mitotic cycles characterized by a long growth period followed by two or more divisions in rapid succession. At least four daughters are produced per division cycle. In both algae, irradiation or plating on actinomycin D (AMD) agar results in a blockade of the cycle in the single-cell stage. Most cells undergo a complete cycle before this occurs. Colonies growing on dithiodiglycol agar are frequently arrested in the two-cell stage. The incorporation of inorganic phosphate into Chlamydomonas DNA is not inhibited by irradiation or by AMD. Incorporation of 14 C into DNA from CO 2 was increased in the presence of AMD. Labeling of protein was relatively unaffected. The rate of incorporation of 14 C into RNA was correlated with the cell count, which increased most rapidly in the controls. Under the conditions tested, chloramphenicol, which produces a similar mitotic block, inhibited 14 C incorporation into all three fractions. The critical stage in the division cycle, at which effects of AMD, chloramphenicol, or previously incurred radiation damage are expressed, is probably the period immediately prior to the first mitosis of the cycle.