The relationship between heterocyte frequency, water temperature, nutrient concentrations and densities of Anabaena planktonica was examined in monomictic, eutrophic Lower Karori Reservoir (max. depth 21 m, area 0.034 km2), Wellington, New Zealand. Anabaena planktonica formed blooms and strongly dominated the phytoplankton assemblage during summer. The development of blooms of A. planktonica was associated with the set-up of stratification and increases in surface water temperature, with the peak of the bloom almost synchronous with maximum water temperature. The annual peak in heterocyte frequency preceded the bloom and occurred during a period of intensifying stratification. At this time, there were sustained low concentrations of dissolved inorganic nitrogen (DIN: nitrate-N + ammonium-N) and the lowest ratios of DIN to soluble reactive phosphorus (SRP), though ratios of total nitrogen to total phosphorus (TN:TP) were more variable. The apparent increase in nitrogen fixation capacity, based on increased heterocyte frequency, appears to be an important prerequisite to bloom formation for A. planktonica in the Lower Karori Reservoir. Monitoring heterocyte frequency may be a useful predictor for the occurrence of blooms of heterocytous cyanobacteria in eutrophic lakes due to the asynchronous nature of development of nitrogen limitation, heterocytes and vegetative cells.