Members of the genus Sargassum can be variable both inter- and intraspecifically. Original descriptions are often vague and open to interpretation, are frequently without accurate illustrations, and may even be based on parts of one or a few specimens, with no account of seasonal variation. This paper presents the results of a detailed qualitative and quantitative morphometric study of three species of Sargassum from Reunion Rocks, KwaZulu-Natal, South Africa. Selected taxonomically important traits were investigated for temporal and interthallus variability using coefficients of variation as estimators of variation/stability. Potentially useful, stable traits are highlighted. The following traits were found to be of value in distinguishing the Sargassum species examined with relatively low levels of overall instability (viz. S. elegans Suhr, Sargassum incisifolium (Turner) C. Agardh and Sargassum sp.1): Primary lateral diameter, blade length, blade width, main axis diameter, branch/blade spacing and blade length:width ratio. Main axis length and the number of branches per main axes showed a relatively high overall degree of instability.