Populations of Ulva growing in southern Australia were examined to ascertain the taxonomic value of the range of morphological and anatomical characteristics. Cell shape and chlorplast position, as seen in transverse section of the basal region of the thallus, proved to be non-viable characters which separate the species from each other. In culture, the morphology of mature plant is constant and characteristic for each species and thus confirms the validity of the species as defined by anatomical differences. Five species of Ulva (Ulva fasciata Delile, Ulva lactuca L., Ulva laetevirens Areschoug, Ulva rigida C. Agardh, Ulva stenophylla Setchell et Gardner) occur in southern Australia. Ulva rigida C. Agardh and Ulva rigida sensu Bliding, through morphological similarities, are distinct species and the latter is referred to as U. laetevirens. The two species show marked differences in diagnostic anatomical characters and in the pattern of development in culture. Two taxa, Ulva australis Areschoug and Ulva spathulata Papenfuss, are included within U. rigida. Records of Ulva latissma L. in this geographical region probably refer to the broadly expanded morphological forms of U. rigida and U. laetevirens. The status of Phycoseris ulva Sonder, described from the West Australian specimens, is unresolved.