1. On artificial substrates, placed in ditches of the Hortus Botanicus, succession was not observed within a period of six months; periodicity phenomena however, could be indicated. Possibly succession becomes evident only over a longer period of time. 2. When studying algal communities on artificial substrates a colonization phase has to be taken into account. In our investigation this period lasted about ten days. 3. The colonization period is easily calculated from production measurements. 4. The nature of the substrate influences the production level of the communities growing on it.