The rimoportula is a morphological feature that is particularly common among centric diatoms but which has not featured prominently in descriptions of the many species of Aulacoseira. Primarily this is because it is often difficult to observe with the light microscope but using very simple techniques and scanning electron microscopy it has been found in most species investigated. We report its variation in form, number and position on the valve mantle and ringleist. It promises to be useful in species determination, taxonomy and in evolution studies. Why it should be absent in at least one species is, at present, an enigma in terms of the functioning of the organelle.