The effects of N and P limitation on cell division and on cellular contents of limiting and non-limiting nutrients were examined in cyclostat cultures of the freshwater diatoms Asterionella formosa and Fragilaria crotonensis. Nutrient-limited cell cycles of both species were readily entrained by a 14 h/10 h light/dark cycle. In F. crotonensis, the maxima relative to the onset of the light phase of the photocycle occurred at approximately -1 to +1 h for cell division, + or - 11 to 13 h for cell N, and +21 to 23 h for cell P. In A. formosa the maxima occurred at approximately +3 to 10 h for cell division and +2 to 4 h for cell N and cell P. Changes in culture dilution rate (integrated growth) and in the type of nutrient limiting growth did not significantly alter these times for either species.