Transparent exopolymer particles (TEP) exist abundantly in oceans and lakes and have been found to play an important role for sedimentation and biochemical cycling of matter. But the origin of these particles and the factors regulating their formation are not well understood. This study examined several strains of algae and bacteria with respect to their production of TEP or TEP-precursors. The formation rate of TEP in batch cultures of algae varied widely between species, and interspecies variability between diatoms was as large as that between species belonging to different classes or even divisions. Species, growth phase and environmental factors acted in concert in determining the accumulation of TEP in algal cultures and no general rules or patterns could be derived. The concentration of TEP during the growth phase of algal batch cultures, mesocosm or natural phytoplankton blooms was a significant function of chl. a, confirming the significance of phytoplankton for the formation of TEP. Experiments with three bacterial strains and a natural bacteria population indicated, that bacteria are also able to generate TEP, but the role of bacterial derived TEP for in situ TEP concentrations remains unclear.