The occurrence of bluegreen algae in saturated salt solutions has been known for a long time. The importance of the algal mat for the salt-industry (2) was recognized long before the organisms themselves were observed. From the scattered literature it appears that no sufficient distinction has been made between the forms that are able to withstand the high concentration of electrolyte and organisms that are able to grow and multiply in such concentrations. It appears that, while the number of resistant species (survivors, or halotolerant organisms) may be large, the group of the truly halophilic bluegreens is very limited. For various samples of salt and brine, obtained from widely scattered localities, nearly always yielded the same form. Truly halophilic forms are considered those that are able to develop in solutions more concentrated than 3 Molar (± 17.55 %) NaCl. The form which reoccurred in the majority of the salts investigated proved to be extremely variable, so much so that adjacent cells often seemed to belong to different species. Inasmuch as the classification of bluegreen algae is notoriously difficult and as it proved impossible to isolate single cells and culture them the question as to the specific identity of the forms remains doubtful. The occurrence of transition-stages seems to point to a great variability, similar to the variability in the purple bacteria as described by Van Niel (15). The presence of "giantcells” is especially typical for the strong brines. The first author of this paper is therefore inclined to consider most of the forms observed as belonging to one species: Aphanocapsa litoralis Hansgirg. (Aphanothece halophytica Fremy, Dzensia salina Woronichin).