Abstract Persistent mass occurrence of the toxic cyanobacterium Oscillatoria agardhii Gomont is reported from Östra Kyrksundet (area 200 ha, max. depth 22 m, mean depth 8.5 m), a eutrophic lake and freshwater supply on Åland Islands, SW Finland. In August 1987 O. agardhii and a peptide hepatoxin characteristic of this species (as analyzed by HPLC) co-occurred in the upper 10 m layer. The toxin and O. agardhii were still present in the lake after the autumnal overturn and even under the ice in March 1988. The highest toxin concentration (37 μg/1) and the highest chlorophyll α values (35 μg/1) were recorded at 6 m near the water intake level on 13 August 1987, when the health authorities stopped the use of the lake as a drinking water reservoir. Several limnological features of the lake, e.g. stratification, low transparency, high pH and a fairly high N/P ratio, favor O. agardhii. Persistent blooms of toxic cyanobacteria imply considerable water quality problems and health risks and may also lead to an impoverishment of the fauna in lakes. The observations in Lake Östra Kyrksundet and previous work on eutrophic freshwater reservoirs (with fish and bird kills associated with cyanobacterial blooms) raise some general questions concerning water quality criteria and the monitoring of toxic cyanobacteria in lakes and reservoirs.