Aim: To study the relationship between the nature of the substratum and the diversity and stability of the ammonia- oxidizing microbial community in a constructed wetland for the treatment of wastewaters. Methods and Results: Samples have been taken the year around from sections of the wetland filled with different substrata. When present, the root zones of the helophyte Phragmites australis were also sampled. The diversity of the ammonia- oxidizing community was established by a coupled PCR DGGE method based on the 16s rRNA gene. Averaged over the seasons, no large differences in community composition were observed between the different substrata, although the section with zeolite always showed the highest frequencies of bands belonging to ammonia-oxidizing bacteria of the beta- subclass of the Proteobacteria. Only sequences related to the Nitrosospira lineage were detected. Averaged again over the seasons, the section with zeolite was also most constant with respect to the potential ammonia- oxidizing activity. Conclusions: Although the ammonia- oxidizing communities did not differ significantly between the different sections of the constructed wetland, the characteristics of zeolite were most appropriate to accommodate a stable and active community of ammonia- oxidizing bacteria. The presence of the helophyte had no effect on the diversity and stability of the ammonia- oxidizing community. Significance and Impact of the Study: It has been shown that substrata used in constructed wetlands made no distinction between ammonia- oxidizing strains in relation to attachment. However, zeolite had the best performance with respect to activity over the seasons.