Charophytes produce a thick-walled zygote, the so-called oospore, the characters of which (size, shape, and structure) are used as taxonomic determination criteria. In the present study, the variation of length, width, length-to-width ratio, and number of striae of oospores collected both from the field and after cross-fertilization experiments was compared within and among taxa, populations, and individuals belonging to the Chara baltica Bruzelius “species complex.” Although the oospore characteristics differed significantly among the taxa, the variations among populations belonging to the same taxon and even among individuals belonging to the same population were still higher. Oospores developed by means of allogamy were not significantly different from oospores developed by autogamy. Oospores were 5% shorter and 27% narrower when measured dry compared with wet material. Information about this treatment is unfortunately lacking in charophyte monographs and oospore determination keys. We concluded that oospore descriptions of different charophyte taxa should be based on a larger amount of data material collected from different populations and individuals and accompanied by a detailed method description, and that a determination of closely related taxa by means of oospores should be interpreted carefully. Ecological aspects of oospore size variation are discussed. [ABSTRACT FROM AUTHOR]