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During summer 2001, periphytic algae associated to the giant bulrush Schoenoplectus californicus (Cyperaceae) were studied in Los Padres and La Tapera streams (Pampa Plain, Argentina). One hundred and seven taxa were determined, being 76 of them common to both sampling sites. Diversity, total number of taxa and abundance of periphytic algae were greater in stems from the effl uent (La Tapera Stream). Diatoms were the most important group in both streams, according to their richness and abundance. Periphyton from the infl uent (Los Padres Stream) was characterized by the dominance of the diatom Navicula cryptocephala and the codominance of another three algae. In contrast in La Tapera Stream, not any species could be recognized as dominant. Periphyton community architecture was more complex in the effl uent, with a high development of fi ve physiognomic groups. In conclusion, algal community attached to S. californicus differs in terms of diversity, abundance and community complexity, possibly due to the differences registered in certain abiotic parameters, mainly in water transparency and velocity