Abstract This publication reports on the chemical and phytoplankton aspects of a three month biological survey of Nyumba ya Mungu reservoir, Tanzania. The lake had a total salt concentration of about 8 meq dm −3 and specific conductivity of 900 μS cm −1. The main salt in solution was sodium bicarbonate, and when the lake level fell it sometimes formed a crust on the exposed shore. The concentrations of major nutrient ions were probably not limiting to algal growth and good nutrient replenishment was provided by the two inflow rivers. Nyumba ya Mungu supports a rich phytoplanton dominated by Melosira and blue-green algae. Acetone extractions gave chlorophyll ‘a’ concentrations ranging from 20–40 mg m −3 in the vertical profiles, and light and dark bottle experiments indicated a maximum gross photosynthesis of about 800 mg O 2 m −3h −1 There was evidence of chemical stratification in the open water and diurnal stratification in the sheltered bays, but discontinuities were short-lived and the Trade Winds ensured regular mixing of the water. The lake appeared to have a higher primary production than the larger Rift Valley lakes and it was considered that this level of production would continue.