Immunofluorescence assays using polyclonal and preadsorbed antibodies as well as FITC-labelled lectins were employed to characterize two morphospecies of the cyanophytes Pseudanabaena catenata and P. planctonica from laboratory cultures isolated from two reservoirs in Spain and one in Denmark. Binding of both antibodies and lectins was unaffected by phase of the cell division cycle, growth phase and environmental factors such as culture medium, light or temperature. Good-quality polyclonal antibodies were obtained for several strains of both morphospecies. As expected, different morphospecies from the same location do not share common antigens while strains of the same morphospecies grown in different reservoirs do share most of their surface antigens. However, P. catenata grown at La Pinilla (Spain) does not share common antigens with P. catenata from Bastrup (Denmark). Geographical barriers, in this case, seem to play an important role in determining surface antigen diversity. Similar results are obtained when lectins are used, and some lectins are found to be useful tools for differentiating clones of the same morphospecies. From a genetic point of view there may be no clear species boundary for some morphospecies of cyanobacteria. A ‘fuzzy species’ concept, i.e. a species that belongs in part to more than one species or has characteristics of more than one species, is speculatively considered to explain genospecies boundaries of cyanobacteria.