The biological and economic importance of the marine Dinoflagellata, as well as the unique taxonomic position of the group, call for a thorough investigation of their metabolic relations. As a preliminary to the wider problem presented by the planktonic genera, the shore-living forms are being studied with a view to determining their metabolic processes and the physical and biological factors which influence their relation to the general nexus of living forms in the tidal area. The present report deals with salinity as a determining factor in the distribution and photosynthetic activity of Amphidinium herdmani, Kofoid, one of the more abundant dinoflagellates in the Port Erin area. It is shown, experimentally, that the organisms, suspended in water and exposed to diffuse daylight, are capable of assimilating free carbon dioxide, at all salinities, from that of normal sea-water down to practically fresh-water. The rate of assimilation, however, is low when the salinity lies outside an optimal range (4 to 8 per mille), and it is possible that under these conditions Amphidinium may function, facultatively, as a heterotrophic organism. Determinations made upon samples of interstitial water, taken on the beach, demonstrate that salinity is a factor--there are doubtless others--in the distribution of Amphidinium, and that the strongly discoloured areas, where it is present in great profusion, are characterised by salinities lying within the optimal range indicated above.