Ctenocladus circinnatus Borzi is a filamentous green alga restricted to highly saline solutions where Na is the dominant cation. Investigations on the influence of various salt solutions were conducted primarily on akinetes, as this stage is important in maintaining populations of Ctenocladus in extreme natural saline solutions. The Na/Mg ratio and/or Na/total cation ratios are probably more significant in growth and development of Ctenocladus than the monovalent/divalent total cation ratio. Vegetative cells of Ctenocladus were much reduced and irregular when cultured in solutions with Na/Mg ratios below 1.3. Increasing Na/Mg ratios showed normal growth and development with cells possessing a typical laminate chloroplast. The Na/Mg ratios most successful for culturing Ctenocladus in the laboratory correspond closely to those ratios found in natural solutions with Ctenocladus. Studies also indicate that Na may be a physiological requirement during certain stages of development (i.e. akinete germination) for this alga. Germination of akinetes only occurred in Na salt solutions whereas other major cation salt solutions resulted in destruction of the akinetes.