The genus OsciQatoria is now considered in the restrictive sense used by Rippka et al. (1979). The much broader definition of Oscillatmia formulated in the 1989 edition of Beryl's Manual of Systematic Bacteriology (and by others earlier) included strains or species of rather different trichome morphologies and cell di- mensions (such as members now placed into Gcitkrincma}. How- ever, phylogenetic analyses of some representatives demonstrated the validity of exclusion (Turner, 1997; Wilmotte and Herdrnan, this volume). However, in agreement with a morphology-based classification into five clusters, such comparative studies also sug- gest that further generic subdivisions may eventually be necessary within the current genus Oscillatoria.