ABSTRACT. – The dogma that diatom cell walls are faithfully reproduced without variation from generation to generation is questioned. We suggest that there are many sources of variation in diatom cell wall morphology, and this polymorphism can be size-dependent and size-free. We discuss three major sources of variation, namely, genetic, ontogenetic and environmental, and present a survey of the literature to support our argument. We also suggest that this variation may be adaptive or circumstantial. It is important for those interested in diatom taxonomy, systematics, and ecology to recognize intraspecific variation in diatom taxa, and the implications variation holds for these disciplines. We call for renewed and robust research programs focused on each of the three major sources of infraspecific variation in diatoms.