Periphyton alltogether includes plant and animal species attached to the various types of substrates submerged in water, but which do not penetrate into the substrate (Cover i Harrel 1978). Diatoms are one of the most important and abundant component of this community (Sullivan 1999). The are recognized by the shell called frustulum which is made of quartz - SiO2xnH2O and it represents some kind of periplast modification. In the periphyton there can be found also different species of dinoflagellates which are not firmly attached to the substrate, but move freely throuhg it. They are very important due to their ability to create massive blooms called ˝red tides˝ which affect not only marine species, but also human populations. They produce neurotoksins that affect muscular function of the infected organisms. Studies have already showed the importance of diatoms as primary colonizers on natural nad artificial substrata supstrata (Hendey 1951, Castenholtz 1963, Neushul i sur. 1976, Niell 1979, Santelices i sur. 1981, Hudon i Burget 1982, Edyvean i sur. 1985, Niell i Varela 1984, Delgarado 1989), and that they respond directly to chemical physical and biological changes in the marine environment. Their use in ecological interpretation of pollution and eutrophication induced changes is also very important (Snoeijs 1991). Studies done in the Adriatic Sea are mostly based on the research of epiphytic diatoms in the northern Adriatic (Totti et al. 2007, Munda 2005) and on the ecology and taxonomy of periphytic diatoms of the karstic rivers in Croatia (Caput et al. 2008, 2005, Burić et al. 2004). Southern Adriatic id being still investigated, and no studies have yet been published. On the other side, further studies od dinoflagellates can help us understand and prevent massive toxic blooms of these small unicelullar, but harmful organisms.