The genus Oscillatoria is now considered in the restrictive sense used by Rippka et al. (1979). The much broader definition of Oscillatoria 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 dimensions (such as members now placed into Gcitkrincma}. However, phylogenetic analyses of some representatives demonstrated the validity of exclusion (Turner, 1997; Wilmotte and Herdrnan, this volume). However, in agreement with a morphologybased classification into five clusters, such comparative studies also suggest that further generic subdivisions may eventually be necessary within the current genus Oscillatoria.