The brackish-water Lake Vargsundet (SW Finland) was affected by multiple algal toxins and extensive fish mortality in July 1997. Dense populations of the ichthyotoxic haptophyte Prymnesium sp. and the hepatotoxic cyanobacterium Planktothrix agardhii were observed simultaneously, but usually in distinct layers. Fish died in the relatively clear epilimnion where 10-40 million Prymnesium cells per litre were recorded (chlorophyll a range 6-18 μg l-1). Fish survived in the metalimnion dominated by the hepatotoxic P. agardhii (chlorophyll a 20-40 μg l-1). The deeper layers were anoxic. The Prymnesium-rich water remained ichthyotoxic until the end of August but the toxicity was less marked in tests with Artemia salina. As even inconspicuous haptophyte blooms with low chlorophyll a concentrations may be harmful, safety and monitoring issues need reconsideration.