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1. Changes in species composition of the periphyton on introduced substrates were determined in an oiigotrophic mountain stream subject to long-term heavy metal contamination.2. At the upstream control site, the numerically most abundant taxa were Bacillariophyta {Achnanthes minutissima, Achnanthes microcephala and Achnanthes linearis) as well as, in summer, the Chlorophyta (Mougeotia spp. and Ulothrix subtilissima).3. At the downstream contaminated site the periphyton community was totally dominated by Bacillariophyta throughout the sampling period. A. minutissima and A. microcephala were co-dominants during spring. Seasonal succession patterns did not parallel those at the upstream site. Chlorophyta were virtually absent and A. minutissima comprised 94% of the community during summer.4. Species diversity, species evenness and dissimilarity index were utilized to delect differences in species composition, abundance and number. Slight differences were found in spring samples while summer samples indicated major differences between sampling sites.

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