ABSTRACTCell division in most eukaryotic algae grown on alternating periods of light and dark (LD) is synchronized or phased so that cell division occurs only during a restricted portion of the LD cycle. However, the phase angle of the cell division gate, the time of division relative to the beginning of the light period, is known to be affected by growth conditions such as nutrient status and temperature. In this study, it is shown that the phase angle of cell division in a diatom,Cylindrotheca fusiformisReimann and Lewin, is affected by the N‐limited growth rate; cell division occurred later in the dark period (12:12 h LD cycle) when the growth rate was infradian (D = 0.42 d−1) than when it was ultradian (D = 1.0 d−1). Nitrogen‐pulses did not affect the phase angle of the division gate, but could shift the time of peak cell division activity within the division gate. The effects, if any, of N‐pulses were dependent upon the growth rate and the time of day that the pulses were administered. These responses indicate that the timing of cell division in this diatom is not determined solely by the zeitgeber from the LD cycle, but rather that a LD cycle control mechanism and a N‐mediated control mechanism are both involved and are somewhat interdependent. In addition, an increase in protein was observed immediately after administering a N‐pulse toC. fusiformisin the ultradian growth mode indicating that the accumulation of protein can be uncoupled from the cell d