Light-dark (L-D) synchronized Chlamydomonas reinhardi were X-irradiated (9108 R) early in the dark period as cells were preparing for or undergoing division. Division delay data were recorded both in terms of the entire population of cells and in terms of cells progressing to form either 4- or 8-celled clones. The lack of division delay exhibited by these cells is striking: The greatest delay among 4's was a 2.2-hr delay in their final division after irradiation at the 14th hour. The greatest delays in 8's were 2.3 and 2.4 hr in their final divisions after irradiation at the 13th and 14th hours. Such delays are small compared to delays induced in other cells by exposures leading to comparable survival levels (22% survival).

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