Constructed wetlands have been shown to be capable of removing a wide variety of contaminants, including bacterial pollution. However, only limited information exists on the distribution of bacteria on roots of macrophytes growing in constructed wetlands. Constructed wetland with sub-surface horizontal flow at Nucice near Prague, Czech Republic, was put in operation in 1996. The system treats municipal sewage from 650 PE and the total area of the beds, planted with Phalaris arundinacea and Phragmites australis in alternate stripes perpendicular to the flow direction, is 3,224 m2 (2 beds 62 x 26 m each). Pea gravel (8/16 mm) was used as a filtration material. During the period 1998-1999, distribution of total aerobic and anaerobic bacteria, coliform bacteria and fecal streptococci was monitored in wastewater as well as on roots of both macrophyte species. Counts of bacteria on root surface in the system at Nucice indicate that there is a steep decrease in bacterial numbers within the first few metres of the bed and that there is significantly more bacteria on roots of Phragmites as compared to Phalaris. There was no statistically significant influence of the season on the bacterial counts on roots of macrophytes.