The IJsselmeer (surface area 1200 km 2 , mean depth 4.5 m, residence time 0.4 year, phosphorus load 7 g m −2 year −1 ) is a very important conservation area. Regular summer blooming of Oscillatoria spp. can depreciate this value, so the bounds of possibility of this kind of blooming have been investigated. Therefore samples were taken along the shore and in the open water from 1974 to 1982, continuous temperature profile measurements were made in the same period in the middle of the lake and in situ primary production was measured in 1976 and 1977. The phytoplankton consists of green algae throughout the year, diatoms in spring, and blooms of Microcystis aeruginosa in summer. Blooming of Oscillatoria agardhii Gom occurs regularly in summer along the Frisian shore. In 1976, however, a heavy bloom of this alga occurred in the whole lake. Comparing the IJsselmeer with shallower Oscillatoria -lakes in the Netherlands distinct differences are present concerning biomass, chlorophyll a content, relative volume of the euphotic zone and light-dark cycle. Not only the larger depth and extensiveness of the IJsselmeer are unfavourable factors for Oscillatoria , but also the separation by land reclamation of many of the shallow littoral regions from the main body of the lake. High temperature and microstratification are needed to develop a bloom in the whole lake.