X-Irradiation delays division of heat-synchronized Tetrahymena pyriformis (a ciliate protozoon) if exposure occurs prior to a critical time after the end of the synchronizing treatment (EST). Exposure after this critical time does not significantly delay the first synchronized division but does delay the second. The time of the transition from a state where the cells are delayed to one where they are not is somewhat dose dependent: within limits, the larger the exposure, the later is the transition time. The division delay response is always correlated with a resorption of the oral primordium of the presumptive posterior daughter cell. The amount of time for resorption varies with the time after EST at which the cells are irradiated and is dose dependent. The data are consistent with the hypothesis that irradiation produces damage during all parts of the synchronized cell cycle, but that, depending on the dose and the time of irradiation after EST, the damage may or may not trigger a response that leads to delay of the first division.