A review of previously published hypotheses concerning phylogenetic relationships in the diatom family Achnanthaceae is presented. Data for characters such as valve ultrastructure, chloroplast number and morphology, and sexual reproduction for the genera Cocconeis, Achnanthes and Rhoicosphenia and their purported biraphid allies Mastogloia and Gomphonema are reviewed. Phenetic and cladistic analyses of the character state data suggest that the major genera of the Achnanthaceae may form distinct evolutionary lineages, which have each converged on the reduced raphe condition. Cocconeis appears to be more similar to Mastogloia and to represent one lineage, while Rhoicosphenia appears to be most similar to Gomphonema and to represent a second lineage. The genus Achnanthes may represent at least one additional independent lineage. Taxonomic implications of the present report are discussed.