The ultrastructural features of oospore wall ornamentation patterns revealed by scanning electron microscopy are important taxonomic characters of the Charales. The present study shows inter‐and intrapecific variations in 19 species of the genera Chara, Lamprohamnium, Nitella and Tolypella. This is the first time that the oospore wall ornamentation of Swedish Charales has been documented in detail. In the studied Chara species the ornamentation within species was variable, and partly overlapping between species, but only between closely related species. In contrast, the Nitella species showed distinct differences in ornamentation patterns between species, although the same patterns can be found in different species. This study presents for the first time SEM images of the species Chara rudis (pustular ornamentation), Nitella opaca (pitted ornamentation) and Nitella wahlbergiana (anastomosing network ornamentation). The ornamentation pattern in the Nordic species TV. wahlbergiana, supports its separation from Nitella mucronata which has a reticulate ornamentation. The relationship between length and width of the oospores is also of taxonomic significance. Nitella and Tolypella oospores are roundish, whereas those of Chara and Lamprothamnium are elongate. The extent to which environmental and genetic factors can affect oospore size and shape within a species are still unknown, but in the present study both ornamentation pattern and size provided evidence for the distinction between Chara globularis and Chara aspera. Some further taxonomic problems in the Charales are discussed in the light of the results of this study.