Non—steady—state population growth equations are presented for Isochrysis galbana growing in a nitrate limited chemostat. It is necessary to introduce a more general response function in place of the customary time—lag parameter to describe the observed population growth. The model successfully predicts the population growth over two experiments, one of 56 and another of 80 days, under a variety of dynamic and steady—state growth conditions. The increasing instability at lower growth rates, shown by both the data and the growth equations, indicates that steady—state growth equations may not be adequate in at least some natural environments.