Herewith is a report on possible reasons for low concentrations of cyanobacteria (blue-green algae) cells in the Waikato River in summer 2007-08 compared with previous records from 2003 to 2007. Because of time constraints, this report is a cursory consideration and does not provide the level of detail and analysis required to more fully evaluate phytoplankton succession as it relates to the complex interplay amongst physical, chemical and biological variables. This report was requested by Mighty River Power following a summer when, as a result of drought conditions, the hydro system was operated in a manner similar to 2003, when cyanobacteria blooms(>15,000 cells mL⁻¹) occurred in the upper river and water temperatures were elevated. These operating conditions were characterised by maintenance of high water levels in the hydro dams and low inflows into the system, resulting in relatively long water residence times.