Number and genera (types) of phototrophic red and green sulfur bacteria have been determined in the upper layer of eleven marine sediment samples from the euphotic zone in the Mediterranean Sea. Origin and nature of the sediment samples are given as well as their pH, total nitrogen and sulfide content. The results show that the sediments studied did not provide good growth conditions for Chromatiaceae (Chromatium, Thiocystis, Thiocapsa) and Chlorobiaceae (Prosthecochloris). The total number of these bacteria varied in agreement with the total nitrogen content (organic matter) of the samples; the organic matter content is, therefore, of primary importance for the occurrence of the red and green sulfur bacteria in the samples.