The cell wall of Micrasterias radiosa var. radiosa Ralfs 1848 (Conjugatophyceae) was examined by transmission and scanning electron microscopy. Previous electron microscopy of this taxon has not been performed; thus these are new observations. The cell wall was recognized to be of the Cosmarium type with complex pores external to the plasma membrane that penetrate the secondary wall and with ornamentation arising from the secondary wall. Subdivided regions of the pore apparatus, the pore head, pore bulb, connecting pore channel, and pore depression were detected. Pores of type 4 were located in the isthmal region and at the division of several quaternary lobes. Previously undescribed ornamentation of an asymmetrical swelling on each semicell face was observed. The Cosmarium type cell wall and pores of type 4 are consistent with other investigations upon Micrasterias taxa. The presence of the asymmetrical swelling on each semicell face necessitates taxonomic revision of Micrasterias radiosa var. radiosa.

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