Every surface that is immersed in seawater is rapidly covered by unavoidable biofilm. Such biofilm formation, also known as fouling, is a complex multistage process and not yet thoroughly investigated. In this study, succession of diatoms and bacteria was investigated during a one month exposition on artificial plexiglass (polymer of methyl methacrylate) substrate mounted above the seafloor at 5 m depth. For the biofilm analyzes the fouling was investigated using selective agar plates, epifluorescence, light and electronic microscopy, as well as for HPLC pigment analysis. During biofilm development, the abundance of all biofilm components increased and reached maximum values after one month exposition. In the bacterial community heterotrophic marine bacteria were dominant and reached 1,96±0,79x104 CFU cm-2. Despite the fact that faecal coliforms and intestinal enterococci were detected in the water column, faecal coliforms were not detected in the biofilm and intestinal enterococci appeared after one month of exposition but in a negligible number of 60±10 CFU cm-2. The phototrophic component of the biofilm was dominated by diatoms and reached 5,80x105 cells cm-2, which was supported by pigment analysis with fucoxantin as dominant pigment in concentration up to 110 ng cm-2. Diatom community was dominated by Cylindrotheca closterium and other penate benthic diatoms. A detailed taxonomic analysis by electronic microscopy revealed 30 different taxa of diatoms. The study confirmed that the plexiglass surface in a marine environment is susceptible to biofouling within 30 days of contact. Furthermore, sequence of the colonization process firstly involved bacteria and cyanobacteria, and secondly diatoms, they all together formed a primary biofilm in the sea.