20 samples of phytoplankton were collected by the 'Thalassa' on the Libyan and east Tunisian coast, from Nov 13th to 29th 1969. The vegetal plankton is rather poor on the whole, except in the Gulf of Gabes. Diatoms are dominant and Rhizosolenia plays a leading part. Among the Dinophyceae, the great variety of Ceratium underlines the subtropical affinities of the study area.