Freshwater and/or brackish water species [127] of the generally regarded marine planktonic diatom genus Thalassiosira [T. visurgis, T. baltica, T. incerta, T. simplex, T. lacustris, T. weissflogii, T. pseudonana, T. gessneri and T. salvadoriana] are known. The morphology based on light and EM, taxonomy and distribution of 6 of them are discussed. The new combinations T. faurii [comb. nov. Coscinodiscus faurii Gasse] and T. rudolfii comb. nov. [C. rudolfii Bachmann] are proposed, and T. guillardii sp. nov. including Guillard''s clone 7-15, is described. While the areola array is of the same type, the process pattern of these species, particularly the number of labiate processes and the number and arrangement of central strutted processes, varies greatly.