Alternative strategies for the dark assimilation of ammonium and nitrate into microalgae are explored using a mechanistic model of algal physiology. The standard diatom strategy, continuation of N assimilation at high rates in darkness as long as reserve C remains, is the most advantageous. The flagellate strategy, incorporating ammonium but not nitrate at a reasonable rate in darkness, is best suited to organisms with high metabolic costs, inhabiting waters with relatively high concentrations of ammonium. The strategy of vertically migrating diatoms - accumulation of nitrate in internal pools for assimilation after return to the photic zone - is best suited to slow-growing cells in low-ammonium environments. Differences between the strategies become less significant with increasing N-source limitation (the situation more typically encountered by flagellates and migratory species) because transport rather than post-transport assimilatory processes become most limiting. It is suggested that optimization of dark N-assimilation is not a critical selective feature; organisms with contrasting abilities in this regard usually inhabit different water bodies and have other more fundamental phenotypic differences (e.g. motility or silicon requirements).