Organic-walled dinocyst morphology is shown to be controlled by changes in salinity and temperature in some species, more particularly process length variation. This is known to be the case for " Lingulodinium machaerophorum " from culture experiments, and study of surface sediments. Also variations in the morphology of the species " Operculodinium centrocarpum " can be related to salinity and/or temperature. Also cysts of the species " Gonyaulax baltica " shows morphological variations in culture, as well as "Gonyaulax spinifera". Cyst formed by other species such as "Pyrophacus steinii" (cyst is called "Tuberculodinium vancampoae") do not show a clear relation to variations in salinity.