It is not so very long ago that botanical concensus abandoned the distinction that was drawn between Algae and Flagellata, the latter having been regarded by zoologists as a group of the Protozoa. Even today some colourless flagellates may find themselves occupying different locations in contrasting taxonomic schemes. However, modern developments in ecology have placed greater emphasis on functional attributes of organisms, and hydro biologists have certainly been in the forefront of such changes. Thus we find that flagellates are again the subject of controversy, not so much over their taxonomic position as over their functional position - their role in aquatic ecosystems. At the 22nd Congress of the International Association of Theoretical and Applied Limnology in Lyon, France in August 1983, we conceived the idea of a meeting to discuss 'Flagellates in Freshwater Ecosystems'. This idea was approved by the committee of SIL and we arranged for the meeting to take place during June 1986 at Lammi Biological Station (University of Helsinki) in Finland. Participants from nine countries enjoyed outstanding hospitality from the staff at the Station during a week of lectures, excursions and discussion ses­ sions. On behalf of the participants we express our thanks for the generous sponsorship received from the Maj and Tor Nessling Foundation, the Academy of Finland, the Finnish Ministry of Education, Lammi Biological Station and the Finnish Limnological Society.

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One: Taxonomy and Life Histories of Flagellates.- Records of chrysophytes with siliceous scales (Mallomonadaceae and Paraphysomonadaceae) from Finnish inland waters.- Variability in size and ultrastructure of the statospore of Mallomonas caudate.- Sexual reproduction and life cycle of Ceratium furcoides (Dinophyceae) in situ in the lake Plußsee (West Germany).- Morphology and life cycle of Ceratium rhomvoides nov. sp. (Dinophyceae) in the lake Plußsee (F. R. Germany).- Two: Spatial Distribution and Motility of Flagellates.- Lake Chisholm, a polyhumic forest lake in Tasmania.- Horizontal mesoscale distribution of phytoplankton in the Tvärminne sea area, southern Finland.- Vertical distribution and diel migration of flagellated phytoplankton in a small humic lake.- Seasonal variation in the diel vertical distribution of the migratory alga Cryptomonas marssonii (Cryptophyceae) in a small highly humic lake.- Vertical migration patterns of flagellates in a community of freshwater benthic algae.- Some size relationships in phytoflagellate motility.- Three: Seasonality of Flagellates.- Population dynamics of Ceratium spp. in three English lakes, 1945-1985.- Spring development of a Chlamydomonas population in Lake Nimetön a small humic forest lake in southern Finland.- Some aspects of the seasonal distribution of flagellates in mountain lakes.- Seasonal occurrence of silica-scaled chrysophytes under eutrophic conditions.- Four: Trophic Relations of Flagellates.- Seasonal and spatial distribution of cryptophycean species in the deep, stratifying, alpine lake Mondsee and their role in the food web.- Flagellate grazing on bacteria in a small dystrophic lake.- Grazing of bacteria and phytoplankton by heterotrophic nanoflagellates in a Baltic Sea sample.- Five: Flagellates and the Environment.- Mass development of the flagellate Gonyostomum semen (Raphidophyceae) in Swedish forest lakes -an effect of acidification?.- Chrysophyte scales recorded in lake sediments from eastern Finland.- Phytoflagellates and their ecology in Tasmanian polyhumic lakes.- Phytoflagellates and their ecology in Finnish brown-water lakes.- Flagellates in freshwater ecosystems - Concluding remarks.