The ultrastructure of the pyrenoid in the gametophytic phase of the species of Ulothrix shows a large diversity. This was revealed by a study of the morphology of the pyrenoid matrix, the associated starch cap and the pattern of the intrapyrenoidal chloroplast strands. From these features, eight types of pyrenoid structure were recognized in the genus Ulothrix. The closely related species Ulothrix crenulata and U. verrucosa, which only propagate by fragmentation, also have a common, very characteristic pyrenoid morphology. In contrast, Ulothrix mucosa, only capable of reproduction by means of zoospores and possessing a peculiar soft cell wall, has a pyrenoid structure overlapping with that in the not very closely related U. zonata, which is characterized by a complete life history and a firm cell wall. The findings reported here support the recently proposed classification of the genus Ulothrix, especially that of the problematic group of species having cell diameters ranging from 3.5-12.6 pm.