Currently, blue-green algae are classified as either freshwater or marine depending on the ionic requirements of the strain, not on the type of habitat from which the strain was isolated. As a result many strains isolated from saline environments are classified as freshwater strains. New parameters were sought which might correlate better the physiology of marine strains with their habitat. This study reveals that blue-green algae isolated from the marine environment have a unique osmoregulatory system which distinguishes them as a coherent physiological group, distinct from blue-green algae isolated from non-marine habitats. The "marine" blue-green algae can be identified by their ability to synthesise and accumulate 2-O-α-D-glucopyranosylglycerol (glucosylglycerol), a major osmoregulatory compound, and by their related ability to grow in seawater-based medium with total maximum NaCl of 6 to 11% (w/v). These two properties allow a more rigorous definition of "marine" than the current method of classification.