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Four colonial species of the multipolar centric diatom genus Bacteriastrum – B. furcatum, B. hyalinum, B. jadranum and B. mediterraneum – were investigated using field samples from the Adriatic Sea and cultured strains obtained from these samples. Morphology and frustule ultrastructure were examined using light and electron microscopy, and nuclear-encoded large- subunit (LSU) rDNA sequences were obtained from the strains. The description of B. jadranum has been emended based on a novel mechanism of colony formation in Bacteriastrum in which cells are joined through an organic cell jacket which holds together cells in chains. Descriptions for two of the three other species were supplemented ; vegetative cells of B. hyalinum have short Y spinules on the fused part of the setae and T-shaped outgrowths, and the spores of this species possess a ring of puncta and a granular surface on the exterior side of the primary valve mantle. Specific for B. furcatum is the location of pores scattered around the setae bases. No unique ultrastructural character was found for B. mediterraneum. Phylogenetic results grouped the sequences of the four species in a clade in which B. furcatum (section Sagittata) and B. hyalinum (section Isomorpha) formed a clade as sister to a clade with B. jadranum (section Isomorpha) and B. mediterraneum (section Sagittata). Apparently, neither section was monophyletic, and the supposed defining features of isomorphic terminal setae in Isomorpha and dimorphic ones in Sagittata were not synapomorphies. Nonetheless, the genus Bacteriastrum itself remained monophyletic.