Phytoplankton samples usually represent heterogeneous assemblages of various forms of algae, which differ widely in their taxonomic relationships and biological complexity, each member having its own individual ecology and physiology (cf . CHU, 1942CHU, , 1943 ; ; RODHE, 1948 ; KETCHUM, 1954 ; etc .) . In order to get a clear understanding of the biology of these natural populations, studies on their nutritional requirements in the laboratory are necessitated by growing them in pure or at least unialgal cultures using synthetic culture media . CHU (1942), taking into consideration the composition and degree of dilution of the natural waters, prepared suitable media for the growth of various planktonic algae using pure chemicals, the formulae being used on the results of a series of experiments and on the analyses of freshwaters . RODHE (1948), with his culture solution VIII, cultivated a number of different plankton algae, and he could not obtain a single plankton alga belonging to Cyanophyceae, Chrysophyceae, Cryptophyceae or Dinophyceae in pure culture or on a completely synthetic media . FOGG (1957) pointed out that a few truly planktonic algae have been isolated in pure culture . KETCHUM (1954) reviewed and classified the various standard synthetic media to indicate the range of concentrations, which have been used by various workers. He agreed with PRINGSHEIM (1946) in that there was no general agreement concerning the best medium, nor do the media designed for different groups of algae vary more than the media designed by different investigators for the same alga .
With the object of finding out suitable media for the plankton algae of the river Ganges, six forms -Ankistrodesmus falcatus, A . convo- lutus, Dictyosphaerium ehrenbergianum, Coelastrum sp ., Spirulina platensis and Fragilaria crotonensis -were isolated from plankton and cultured in different culture solutions .