Periphyton spamles from the reed-belt and phytoplankton samples were taken in November 1996 and in January, April, July 1997 at the upper part of the Soroksár-Danube (the second largest side arm of river Danube in Hungary) at Taksony, and in July, November 1998, January, April 1999 at the middle part of the side arm at Ráckeve. The aim of this study was to investigate the seasonal and vertical distribution of periphyton growing on old and green reed stemps at both parts of the side arm, focussed on taxonomic composition, abundance and chlorophyll a content. Based on the chloropyll a content of the phytoplankton the upper part of the side arm at Taksony was oligotrophic in November an July (in July caused by a flood), mesotrophic in Januray and eutrophic in April. The middle part of the Soroksár-Danube was eutrophic in April and July, oligotrophic in November and January at Ráckeve The values of abundance and chlorophyll a of periphyton were the highest at the middle part of Soroksár-Danube, while that of phytoplankton was the highest at the upper part. A few diamonts showed the same distribution along the reed stems. Relative abudance of Amphora libyca, Cocconeis placentula and Eunotia arcus increased close to the bottom, while that of Fomphonema minutum, G. paravulum, Navicula capitatoradiata and Nitzschia dissipata decreased.