Phaeodactylum tricornutum is a pleiomorphic diatom that exists in three different forms depending on environmental conditions, viz. oval, fusiform or triradiate. In spite of numerous observations of the three morphotypes, very little information is available about the process of transformation. In this review, we summarize results of previous studies and add new observations on cell wall structure of the three morphotypes and the environmental conditions under which each form appears ( for different strains of Phaeodactylum). New data on the cellular mechanisms of transformation between the three morphotypes and on the differentiation of oval cells are also presented. The ultrastructural morphology of the three morphotypes and strain/genotype-dependent transformations of morphotypes are discussed.