A study was made on morphological and cytological features of four Rivulariaceae strains from marine, partially saline and freshwater environments using three versions of saline and freshwater media (standard. - Fe. low P). A brackish Anabaena sp. was used for comparison. Subculture to - Fe medium led to marked changes in morphology and cytology. These were (base to apex): a series of new heterocysts apical to the original one. Intra-thylakoldal vacuolation. Lack of cyanophycin granules, presence of polyphosphate granules, pale colour, brown sheath, short hair and low hairiness. With low P medium, the changes were the opposite. Hairs did not form in Calpthrix D253 in saline medium, whether the cultures were in - Fe or low P medium. Downshift from saline to freshwater medium led to the synchronized development of hairs in most trichomes. Whereas the reciprocal upshift led to loss of hairs. The Fe content was studied in three Rivulariaceae and Anabaena in batch culture in high and low P media, which eventually led to Fe- and P- limitation. respectively. The content reached a maximum by 24 h in all strains, decreasing subsequently from 1.80 - 0.06 % dry weight. Siderophore production was tested in four Rivulariaceae and Anabaena in saline and freshwater media. Two Calothrlx strains and Anabaena produced a siderophore in freshwater medium. Influence of Fe status on nitrogenase activity was tested in two Rivulariaceae and Anabaena in saline and freshwater media during batch culture. Peaks of nitrogenase activity in the former came three to five days after the Fe peak, by which stage Fe content had dropped to within the range 0.6 - 0.3 % dry weight. In contrast, peak activity in Anabaena came within a day of the Fe peak. Addition of Fe to Fe-limited cultures led to rapid increase in activity. The significance of these data on morphology, cytology and physiology and possible interactions between Fe- and P-limitation are discussed.