Harpacticoids made up 19 and 30% of the meiobenthos proper by number and 39 and 55% by biomass at maximum depths and a depth of 20 m, respectively, in Lake Päijänne. Harpacticoids accounted for higher proportions of the total meiobenthos in oligotrophic and unpolluted areas than in polluted areas. Seven species were identified, with Canthocamptus staphylinus confined to semi-lotic areas influenced by organic pollution from a pulp mill. Other species exhibited somewhat different distributions in oligotrophic and mesotrophic areas. Harpacticoid densities were most closely correlated with oxygen content and phytoplankton biomass, and correlations with environmental values were closer at maximum depths than at 20 m. Attheyella crassa densities in the deepest zone had most marked negative correlation with total phosphorus, phytoplankton biomass, dissolved organic matter and organic sedimentation, while Paracamptus schmeili had the closest positive correlation with oxygen content. High densities of these species could be classified as an indicator of oligotrophic conditions. Similar correlations, although weaker, were evident for Bryocamptus minutus and still weaker for Bryocamptus echinatus and Moraria brevipes. Clustering of the stations based on harpacticoid abundances gave a similar result to clustering based on environmental data, particularly in the 20 m zone.