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The chemistry of 53 lakes at various stages of acidification and inhabited (at the presence and/or in the past) by pelagic Crustacea was studied in September 1984. Ten of these lakes were investigated in detail biannually (July and October 1987-1990). The July results reflect the influence of snowmelt and were compared with the October ones. The most important anion was sulphate with the average values of 98 and 104 peq 1-1 in 1984 and 1987-1990, respectively. High concentrations of nitrate (21-56 Ieq 1-) were observed in lakes above the treeline. Mean relative composition of cations does not differ between July and October; small changes are in the mean relative composition of anions. Acidification of lakes, expressed as a decrease in alkalinity, is 100 /Ieq 1-, and is equal to the increase in the sum of sulphate and nitrate. The values of total phosphorus and COD are the lowest in the range of pH 5-6.5. Alkalinity, sulphate, nitrate and pH do not show any trend with time over the last ten years.

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