A mass development of Chrysochromulina parva Lackey - 614000 cells per
ml, associated with fish mortality, is reported from a small Danish l
ake. None of the analyses performed showed any reason for the fish kil
l. Even oxygen conditions were favourable. Thus toxins from the Chryso
chromulina might be responsible for the fish kill. If true, this would
be the first record of toxicity in a freshwater Chrysochromulina.